Russia has announced naval war games in the Black Sea and Ukraine has put its troops on combat alert, as the United States called on both sides to show restraint amid dramatic new tensions over the Crimea Peninsula.
The moves on August 11, coming one day after Moscow claimed it thwarted an incursion into Russian-held Crimea by Ukrainian saboteurs, stoked fears that in Ukraine and elsewhere Moscow may be planning to ramp up fighting in a war between Kyiv and pro-Russia separatists in eastern Ukraine.
Russian President Vladimir Putin used some of his most aggressive rhetoric since the height of the war two years ago, pledging to take countermeasures against Ukraine, which he accused of sending saboteurs into Crimea to carry out terrorist acts.
In Washington, U.S. State Department spokeswoman Elizabeth Trudeau called for the avoidance of any actions that would escalate an already "very dangerous situation."
"We continue to remain in close touch with international partners on this," she said. "But we believe now it's the time to reduce the tensions, to reduce the rhetoric and get back to talks."
"Crimea is part of Ukraine and is recognized as such by the international community," she said.
Earlier, the U.S. ambassador to Ukraine said Washington has seen no evidence supporting the incursion claim by Russia, which accused Kyiv of plotting "terror" in Crimea and reported that two members of its security forces were killed in clashes last weekend with the alleged assailants.
"Russia has a record of frequently levying false accusations at Ukraine to deflect attention from its own illegal actions," Geoffrey Pyatt said.
Following a request from Kyiv, the United Nations Security Council discussed the rising tensions in Crimea during a closed-door meeting in New York.
After the meeting, Ukrainian Ambassador Volodymyr Yelchenko said Russia had amassed more than 40,000 troops in Crimea and on the Ukrainian border.
Russia's UN ambassador, Vitaly Churkin, meanwhile, described the Security Council meeting as "useful" to explain the situation and dismissed concerns about a Russian military build-up.
"Instead of counting our military they should be bringing an end to the conflict [in eastern Ukraine]" he said.
Ukrainian President Petro Poroshenko says he wants to speak directly with Russian President Vladimir Putin and several Western leaders amid the spike in tensions.
Russia's lead security agency, the Federal Security Service, said on August 10 that an agency employee and a soldier were killed over the weekend while repelling what it described as attempts to get Ukrainian "saboteurs" onto the peninsula.
Russia seized and annexed Crimea in 2014 after deploying troops and staging a referendum denounced as illegitimate by Kyiv, Western governments, and a total of 100 countries.
The agency, known as the FSB, also said it had detained several men -- both Ukrainian and Russian citizens -- and confiscated weapons.
Putin accused Ukraine's pro-Western government in Kyiv of using terror tactics to seek to ignite a new conflict and destabilize Crimea, something the Ukrainian president, Petro Poroshenko, called preposterous fantasies and a pretext for Moscow to make more military threats.
In Moscow, Putin held a meeting of his Security Council and discussed “additional measures to provide for the security of the citizens and vital infrastructure facilities of Crimea in connection with the prevention by the [Russian] special services of a terrorist act on the peninsula,” the Kremlin said on its website.
Russian President Vladimir Putin chairs a meeting with members of the Security Council to discuss additional security measures for Crimea on August 11.
On August 11, Poroshenko said he had instructed all military units near Crimea and in eastern regions near separatist-held territory to be at their highest level of combat readiness.
The Russian Foreign Ministry said the deaths of two of its security forces would have consequences, and the Russian Defense Ministry said that the Black Sea Fleet, which is based in the Crimean port of Sevastopol, would conduct exercises on August 11-13 to practice "repelling an attack by underwater saboteurs."
The Russian claims came amid reports of an unusual increase in Russian military activity in the northern part of Crimea, near the administrative border with mainland Ukraine, over the past several days.
That activity, as well as the Russian allegations and the upsurge in fighting in eastern Ukraine, has stoked concerns in Ukraine that the Kremlin may seek to take control of more of its territory.
After annexing Crimea in March 2014, Russia threw its support behind separatists who seized territory in the Donetsk and Luhansk regions, leading to a war that has killed more than 9,500 combatants and civilians since that April. The current tension in Crimea follows weeks of increased fighting in eastern Ukraine.
A NATO official was quoted as saying the military alliance was monitoring the heightened Crimea tensions with concern. The official, who asked not to be named, told the AFP news agency that "Russia's recent military activity in Crimea is not helpful for easing tensions" and called on Moscow "to work for calm and de-escalation."
In Russia, the daily Kommersant cited unnamed sources as saying that two of seven suspects in one group had been killed and the other five captured. Most of them were Crimea residents and some had Russian passports, the report said.
Kommersant also cited the sources as saying the detainees' objective had been to spread panic to destroy the tourist industry, but not to kill anyone.
Ukraine quickly denied the allegations, with military spokesman Oleksandr Motuzyanyk saying Moscow's claims "do not correspond to reality. Ukraine did not commit any armed provocations in [Crimea] or any other area. None of [Ukraine's] Defense Ministry intelligence staff was detained in occupied Crimea."
There was no way to check the veracity of the reports.
Russian authorities frequently report that suspects have confessed, and human rights activists say confessions are often extracted through pressure or torture.
Sergei Aksyonov, the head of the Russian-imposed government in Crimea, told state TV channel Rossia-24 on August 11 that "saboteurs" should be hanged.
"Saboteurs should be treated exactly the same way as farmers treat crows who rob them of their harvest -- they ought to be killed and hanged at the border so the others get the message," he said.
The Digital Forensic Research Lab at the Atlantic Council, a U.S.-based think tank, cast doubt on the circumstances of the incident, saying in a report on August 10 that there were no published photographs or videos of these incidents of gunfire and shelling.
Among those Russia says it has detained is Yevhen Panov, a Ukrainian truck driver who previously volunteered in the conflict zone in eastern Ukraine. Russian state TV published a photograph of a man who appears to be a handcuffed Panov.
Among those Russia says it has detained is Yevhen Panov, a Ukrainian truck driver who previously volunteered in the conflict zone in eastern Ukraine. Moscow claims Panov worked as a Ukrainian military intelligence officer, and Russian television showed footage of Panov being questioned, with several cuts and bruises visible on his face and arm.
TASS reported that a court in Crimea arrested Panov for two months on suspicion of organizing terror attacks on the peninsula.
Kyiv, however, called him a hostage and Panov’s half-brother, Ihor Kotelyanets, told RFE/RL’s Current Time TV that Panov was not a member of Ukraine’s armed services and that he had been abducted.
Kotelyanets said Panov had left his hometown of Energodar on August 6 to visit friends over the weekend and had been due back on August 8.
WATCH: Brother Of Ukrainian Held By Russia Appeals To Kyiv For Help
Another statement questioning the Russian accounts came from the head of Ukrainian military intelligence, who said an armed skirmish had taken place in Crimea between the Russian military and Russian FSB border guards.
Valeriy Kondratyuk did not say when or exactly where the alleged shoot-out took place. He made the comments at a meeting with Poroshenko earlier on August 11 to discuss the crisis.
Other media outlets tried to puzzle out Putin’s motives for the Russian claims.
The independent Russian daily Vedomosti said that Russia may be intentionally increasing tensions and putting pressure on Kyiv and the West ahead of a possible new round of talks on the conflict in eastern Ukraine scheduled for next month.
In an op-ed article titled A New Old Enemy, Vedomosti wrote that Moscow had a habit of ramping up tensions ahead of negotiations with Ukraine.
"The main political question now is what is the future of the Minsk process," the paper wrote, referring to a cease-fire and peace plan that was hammered out in the Belarusian capital in February 2015 but has not been implemented. "Will Russia bring an end to it or demand new concessions?"
Motuzyanyk, the Ukrainian military spokesman, said Russia's actions and statements were intended "to discredit Ukraine, to stall the Minsk agreements, and to escalate conflict in eastern Ukraine which was caused by the Russian aggression."
With reporting by AP, Reuters, dpa, Kommersant, and Vedomosti