Ukraine's unilateral ceasefire hung in the balance Sunday after clashes engulfed the separatist east and Russian President Vladimir Putin put troops on "full combat alert".

The resurgence of violence in the 11-week pro-Russian uprising threatening to splinter the ex-Soviet state came as Washington accused the Kremlin of covertly arming the rebels and sternly warned Putin against sending troops into Ukraine.

A ceasefire announced by Ukraine's new president had barely begun when residents of Andrivka, caught in the crossfire between government and rebel forces in eastern Ukraine, heard mortar fire.

"Nothing has changed," said Lila Ivanova, head of the village council in Andriyivka.

Ukraine's unilateral ceasefire hung in the balance on Saturday as clashes engulfed the separatist east and Russian President Vladimir Putin questioned the new Western-backed leader's claimed commitment to peace.

The resurgence of violence in the 11-week pro-Russian uprising threatening to splinter the ex-Soviet state came as Washington accused the Kremlin of covertly arming the rebels and sternly warned Putin against sending troops into Ukraine.

France and the United States on Friday urged Moscow to help de-escalate the Ukraine crisis or face "new measures" by the Western powers, the French presidency said in a statement.

French leader Francois Hollande spoke by phone with U.S. President Barack Obama and the two called on Russia "to quickly appeal to the armed separatist groups (in east Ukraine) to halt their military actions".

Russia on Friday demanded an apology from Kiev over the "shooting" of a border post from Ukrainian territory that led to the wounding of one customs official, the Kremlin said.

"The Russian side is waiting for an explanation and an apology in connection with this," the Kremlin press office said, according to Russian agencies.

Russia on Friday slammed a week-long unilateral ceasefire announced by Ukraine's new President Petro Poroshenko as an "ultimatum" for rebels battling government troops to surrender.

"A first analysis unfortunately shows that it is not an invitation to peace and negotiations but an ultimatum for the militias in the south-east of Ukraine to give up their weapons," the Kremlin press office said, according to Russian news agencies.

Russia has re-deployed "significant" military forces near its border with Ukraine and stockpiled additional tanks and artillery at a southwestern site, a U.S. official said Friday.

Washington had information that the equipment and forces were being prepared by Moscow "to provide active support for separatist fighters," a senior U.S. administration official said.

Russian President Vladimir Putin and his U.S. counterpart Barack Obama will discuss the situation in Ukraine, Iraq and Syria "in the coming days," a top aide said on Friday.

Speaking to reporters, Putin's top foreign policy adviser, Yury Ushakov, did not provide further details about the upcoming phone call.

Russia has launched a military preparedness drill that includes moving troops in Siberia and the Urals to other locations, a military official said on Friday.

"Some motorized and air defense groups located in the Urals and Western Siberia have been put on alert and have begun multi-kilometer marches... to destination areas," said the assistant commander of Russia's central military district Yaroslav Roshchupkin, Russian agencies reported.

Ukraine's new Western-backed president on Friday released a sweeping peace plan for curbing a pro-Russian uprising in the separatist east that is threatening the ex-Soviet country's survival.

The 14-point initiative's publication followed two calls made by President Petro Poroshenko to Vladimir Putin within 72 hours in the belief that no truce could work without the support of the Russian strongman.