Ukrainian President Petro Poroshenko has announced a unilateral, seven-day cease-fire beginning June 20.
Poroshenko made the announcement while speaking with residents in the town of Svyatogorsk in the Donetsk region.
The Ukrainian Interior Ministry quoted him on its website as saying that "combat action will only be of retaliatory character if rebels attack our forces."
Poroshenko has said a short cease-fire is to be the first step in his plan to ease the conflict.
However, Moscow almost immediately said Poroshenko's cease-fire is not a peace offer but an ultimatum to separatists to lay down their arms.
Russian news agencies quoted the Kremlin as saying Poroshenko's statements lack any proposal on starting negotiations.
The statements comes as Moscow also protested and demanded Kyiv immediately investigate fire upon a Russian border checkpoint in Novoshakhtinsk and the wounding of a customs officer there.
The leaders of the United States, France and Germany, meanwhile, warned Russia that it faces the risk of new sanctions from the West if it fails to defuse tensions on the Ukraine border.
The White House said U.S. President Barack Obama had spoken separately with French President Francois Hollande and German Chancellor Angela Merkel.
Obama, Hollande and Merkel welcomed Poroshenko's cease-fire and urged Russia to pull back its military presence on the border, stop the flow of weapons and militants across the boundary and use its clout to persuade separatists to stand down.
Poroshenko's plan, available on the presidential website, envisions guarantees of security for all parties to negotiations, an exemption from criminal liability for those who did not commit serious crimes and who lay down their arms, and the freeing of hostages.
Other points of Poroshenko's peace plan include the establishment of a 10-kilometer buffer zone at the Ukrainian-Russian border, "the withdrawal of illegal armed groups," and a guaranteed corridor for leaving Ukrainian territory.
Tanks To Separatists
Meanwhile, the Obama administration has concluded that Russia is preparing to deploy “additional” tanks to separatists in eastern Ukraine currently battling the central government.
"We have information that additional tanks departed from a deployment site in southwest Russia yesterday," a U.S. senior administration official told reporters in a June 20 conference call on condition of anonymity.
Ukrainian officials on June 20 briefed EU and G7 diplomats about additional military hardware that Kyiv claims Russia is providing to the separatists, the official said.
The official declined to independently confirm those reports, but said that the United States was "endeavoring to establish the facts through our own means."
Some of the equipment appears to be materiel currently used by the Ukrainian military but not by the Russian military, which could allow separatists to falsely claim that they recovered the hardware on the battlefield and not from Russia, the official added.
The official also said that Russian special forces are maintaining points along the border "to provide support to separatist fighters" and that Russia had "redeployed significant military forces to its border with Ukraine."
Putin had previously ordered Russian forces to withdraw from the border on May 19.
U.S. White House spokesman Josh Earnest said June 20 that the United States is watching the Russian deployment on the border and would not accept any use of Russian military forces in eastern Ukraine.
There was heavy fighting in eastern Ukraine overnight on June 20. Ukrainian officials said seven of their troops had been killed and said that rebels were operating tanks.
Reports said the fighting appeared to be centered around the villages of Krasny Liman and Yampil in the Donetsk region.
One military source quoted by Reuters said some 4,000 separatists had been involved while Ukrainian infantry was supported by some 20 tanks as well as artillery and heavy armor.
Ukraine says 300 rebels were killed, although that figure cannot be verified.
A top rebel commander, Igor Girkin (aka Strelkov), reported "heavy losses" in equipment and arms among the separatists.
In an attempt to put U.S. pressure on the rebels, the Treasury Department announced sanctions on seven individuals stoking unrest in eastern Ukraine.
The individuals are Valery Bolotov, the self-proclaimed leader of the "Luhansk People's Republic"; Igor Girkin (aka Strelkov), who commands the Donbas People's Militia; Valery Kaurov, leader of the Union of Orthodox Citizens of Ukraine; Sergei Menyailo, the acting governor of Sevastopol; Vyacheslav Ponomaryov, the self-proclaimed mayor of the city of Slovyansk; Andrey Purgin, a leader in the self-declared "Donetsk People's Republic"; and Denis Pushilin, the self-declared chairman of the Supreme Soviet of the "Donetsk People's Republic."
It was not immediately clear what assets, if any, the sanctioned individuals have in the United States.
The move stopped short of sectoral sanctions, against whole industries such as gas or oil, which would be seen as a much more aggressive move by Moscow.
Luke Johnson and Carl Schreck in Washington, with additional reporting by Reuters, AFP. dpa, Interfax, and ITAR-TASS