The Ukrainian government said a promised unilateral ceasefire in the east of the country would begin on Friday evening and last a week, as Barack Obama and European leaders discussed imposing new penalties on Russia.

The ceasefire is part of President Petro Poroshenko's plan to win back control over eastern Ukraine, where armed separatists have set up breakaway republics.

Poroshenko travelled to the east on Friday and visited a Ukrainian national guard camp. Wearing military fatigues, he handed out awards to some of the troops, and announced details of the ceasefire plan. "Combat action will only be of retaliatory character if rebels attack our forces," he said.

The White House welcomed the ceasefire plan and said that Barack Obama had on Friday evening discussed imposing new penalties on Russia with German chancellor Angela Merkel and French president Francois Hollande.

The leaders said Russia must pull back its troops from the Ukrainian border and work to persuade separatists to disarm.

The United States is accusing Russia of providing separatists with tanks and heavy weaponry.

The White House says the leaders agreed that if Russia does not take immediate steps to calm tensions in eastern Ukraine, the US and Europe will levy further costs on Moscow.

The ceasefire plan does not envisage negotiations with the leaderships of the so-called Donetsk and Luhansk people's republics, but promises amnesty to all rebels who disarm and are not guilty of major crimes.

It offers decentralisation of power, though it stops short of full federalisation, and gives guarantees over the status of the Russian language, as well as calling for early parliamentary elections. It also calls for a "buffer zone" on the Russia-Ukraine border to prevent the infiltration of Russian weapons and fighters.

Poroshenko's plan is meant to give time for separatists to lay down their arms, and give those who have crossed from Russia safe passage back over the border. But many are locals and in recent days both commanders and rank-and-file fighters have told the Guardian they have no plans to surrender, raising the prospect of an intensified military campaign once the week is over.

In recent days there have been reports that a large number of Russian troops had moved back to the border area, having been withdrawn several weeks ago.

"This is not a matter of some sort of concentration of forces, but of the strengthening of border controls of the Russian Federation," Vladimir Putin's spokesperson Dmitry Peskov said on Friday.

Vladimir Markin, spokesman for Russia's investigative committee, also said on Friday evening that clashes had taken place on the border between Russia and Ukraine that resulted in shots being fired inside Russian territory, and that investigators were being dispatched to the scene.

Putin has so far refrained from sending the army into eastern Ukraine in the same way that happened in Crimea prior to the peninsula's annexation this year, but has repeatedly called on Kiev to stop the military operation in the east. Poroshenko and Putin have spoken by phone twice recently, including once to discuss the peace plan earlier this week, suggesting that some kind of agreement may have been reached between the pair.

How much force any deal would have on the ground, however, is another matter, as many of the myriad groups of fighters do not appear to be under any real chain of command.

Heavy fighting has continued in recent days. Vladislav Seleznev, a spokesman for Kiev government's "anti-terrorist operation", said that seven soldiers were killed and 30 injured in clashes around the village of Yampil. He claimed that 300 rebel fighters had been killed, figures which were impossible to verify. There have also been civilian casualties in towns where the fighting has been most intense, notably Slavyansk, one of the centres of rebel activity and under siege from the Ukrainian army.