Ukraine's president has warned his country there is a "colossal threat" that large-scale fighting could resume in the east, and said the military should be prepared to defend Ukraine against a possible invasion along its border with Russia.
Petro Poroshenko spoke in an annual address to parliament on June 4, a day after the heaviest fighting between government troops and Russian-backed separatist forces in eastern Ukraine in months.
The European Union said the clashes were the biggest breach yet of a February truce agreement and expressed concern they could lead to a "new spiral of violence and suffering."
The EU warned Russia that the future of sanctions imposed on Moscow over its interference in Ukraine would depend on implementation of a tenuous February peace deal known as Minsk II.
Poroshenko said that more than 9,000 Russian soldiers were currently in Ukraine, and that Ukraine's military must be prepared for the possibility of "a full-scale invasion along the entire length of the border with Russia."
The Ukrainian president's warnings were backed by NATO chief Jens Stoltenberg, who on June 4 said that Russia "is present in eastern Ukraine and that it has delivered large quantities of heavy, advanced weapons to the separatists."
The deliveries include "artillery, antiaircraft systems, advanced weapons systems," Stoltenberg told reporters in Oslo. "They have supplied more than 1,000 units of this kind to the separatists."
He added that "Russia has a particular responsibility [to end the conflict] because they back the separatists and supply them with heavy arms. They also have forces in eastern Ukraine."
Ukrainian military spokesman Andriy Lysenko said earlier on the same day that at least five Ukrainian soldiers were killed and 39 wounded in the June 3 fighting with separatist forces near the town of Maryinka, 30 kilometers west of the rebel stronghold of Donetsk.
Lysenko also said that 80 rebel fighters were killed and 100 wounded in the fighting. His claim could not be independently verified.
The Ukrainian military says the separatists, using tanks and artillery, tried to take government positions in an offensive on June 3 in violation of Minsk II.
The rebels blame government troops for the fighting and say 15 civilians and separatist fighters died in shelling by government forces.
But the OSCE's Monitoring Mission said it observed a dozen tanks and other armored vehicles in rebel-held areas, moving "generally in a westerly direction towards the contact line -- close to Maryinka, preceding and during the fighting."
The mission's statement said the Ukrainian military informed the OSCE that it would move heavy artillery to the contact line to deal with the "real threat" posed by the fighting.
Members of the Ukrainian armed forces ride on an armored personnel carrier as they patrol area near Artemivsk in eastern Ukraine on June 4.
The military said on June 3 that Ukraine had warned its "international partners" about its redeployment of the heavy weaponry, which it said was necessary to repel a separatist offensive that involved 1,000 fighters as well as tanks, mortars, and artillery.
Heavy artillery had been pulled back under Minsk II as part of an effort to create a demilitarized zone between government-controlled and rebel-held territory in the Donetsk and Luhansk regions.
Fears For Minsk
The EU voiced strong concern about the fighting and called on all sides to adhere to the cease-fire deal.
"The violent fighting in Maryinka near Donetsk in the east of Ukraine constitutes the most serious violation of the cease-fire arising from the Minsk accords in February," European Commission spokeswoman Maja Kocijancic told reporters.
She added that it followed the movement of a "large quantity of heavy weapons towards the line of contact" by the separatist rebels.
In a warning to Russia, Kocijancic noted that EU leaders had linked future decisions on easing or tightening EU sanctions against Moscow to implementation of Minsk II, which included the cease-fire agreement and other steps to resolve the conflict between Kyiv and the separatists.
"The leaders have made a very clear link between the restrictive measures and the full implementation of Minsk, but they have also said that they remain if necessary [ready to] take further steps," she said. "This remains the EU's position."
France's Foreign Ministry called on June 4 for an immediate end to the fighting.
The Minsk II deal, brokered in the Belarusian capital by German Chancellor Angela Merkel and French President Francois Hollande, has served as the basis for a shaky cease-fire that has decreased fighting but not stopped it.
The fighting on June 3 deepened fears that the Minsk II deal could fall apart entirely, but by midday there had been no reports of major hostilities on June 4.
The conflict has killed more than 6,400 combatants and civilians in eastern Ukraine since April 2014 and further damaged ties between Russia and the West, which had already been severely strained by Moscow's seizure of the Crimean Peninsula from Ukraine in March 2014.
Accusations Of Blame
Ukraine on the one hand, and Russia and the rebels on the other, traded blame for the upsurge in fighting.
On June 4, Kremlin spokesman Dmitry Peskov accused Kyiv of provoking the new fighting to put pressure on the European Union to extend sanctions against Russia. The bloc is set to decide on the matter this month.
"The Ukrainian side has taken steps to aggravate tensions many times in the past in the run-up to some major international events," Peskov said. "This used to happen and we are seriously concerned now over the most recent manifestation of such activity."
Russian Foreign Minister Sergei Lavrov said the same day that the Minsk agreement is "constantly under threat because of the actions of the Kyiv authorities."
Kyiv, NATO, and Western governments accuse Russia of providing arms, personnel, and training to the separatists in the conflict.
Moscow has repeatedly rejected the accusation despite mounting evidence of such involvement.
Meanwhile, the Associated Press reported that U.S. Defense Secretary Ash Carter was summoning top American defense and diplomatic leaders to Germany on June 5 to map out a counterstrategy to Russia's actions in Ukraine and to reassure NATO allies.
AP wrote that the closed-door meeting of about two dozen generals, ambassadors, and other leaders was aimed at assessing how effective the current economic sanctions and U.S.-backed military operations have been in deterring Russia.
With additional reporting by Reuters, AFP, and Interfax