He said this would create conditions for dialogue between the Ukrainian authorities in Kiev and the separatists.

"We're always being told that our forces on the Ukrainian border are a concern. We have withdrawn them. Today they are not on the Ukrainian border; they are in places where they conduct their regular tasks on training grounds," Putin said.

However, a senior Nato official said the Western alliance had not seen any signs of a Russian pull-back from the frontier, where Moscow has massed tens of thousands of troops, proclaiming the right to invade Ukraine to protect Russian speakers.

"We have no indication of a change in the position of military forces along the Ukraine border," the Nato official in Brussels said.

White House deputy spokesman Josh Earnest also said that "to date" there has been "no evidence that such a withdrawal has taken place".

Putin spoke in Moscow after talks with the head of the Organisation for Security and Co-operation in Europe, who said the security and rights body would soon propose a "road map" to defuse the Ukraine crisis.

A pro-Russian separatist leader said the separatists would consider Putin's call to postpone their referendum at a meeting of their self-proclaimed People's Assembly today.

"We have the utmost respect for President Putin. If he considers that necessary, we will of course discuss it," Denis Pushilin said in Donetsk, a city of one million people that the rebels have proclaimed capital of an independent "People's Republic of Donetsk".

Since a pro-Russian president was ousted in an uprising in February, Putin has overturned decades of post-cold war diplomacy by proclaiming the right to send troops to Ukraine and seizing and annexing Crimea.

A rebellion in the east has raised the prospect that Ukraine, a country of about 45 million people the size of France, could be carved up or even descend into civil war, pitting Russian-speaking easterners against pro-European Ukrainian speakers in the West. Residents in areas held by the pro-Moscow rebels were stunned by Putin's remarks at a time when the region seemed to be hurtling towards inevitable independence and a week of bloodshed had brought animosity towards Kiev to a fever pitch.

"Maybe Putin doesn't understand the situation? There is no way this referendum isn't happening," said Natalia Smoller, a pensioner who has been bringing food to rebels manning a roadblock in Slavyansk, a town turned into a fortified redoubt.

"There's no turning back now. We won't retreat. This either ends with our victory or ... it doesn't bear thinking about."

Nevertheless, experts predicted the separatists would heed Putin's call to stand down for now.

"Among those confronting Ukrainian troops, a certain logic should prevail under which they understand that without the support of Russia and thereby the Russian army, they could be subjected to heavy military strikes," said Yevgeny Minchenko, a political analyst known to be friendly to the Kremlin.

Ukraine's government and its Western allies have urgently sought to halt the referendum, which they feared would lead to a repeat of Russia's annexation of Crimea in March but on a much larger scale. US Secretary of State John Kerry has called the proposed referendum "contrived and bogus".

The Ukrainian government troops have launched a military campaign to retake territory held by separatists this week. Troops briefly captured the rebel-held city hall in the eastern port of Mariupol overnight, but quickly abandoned it.

A week of violence in the east and in the southern city of Odessa, where more than 40 people died in clashes, has spread the unrest.