Mr Putin has called for separatists in east Ukraine to postpone a referendum on independence for the mostly Russian-speaking region.

His call comes five days before the vote on secession is due to take place.

Several towns and cities in east Ukraine have come under the control of separatists, who are seeking independence from Kiev.

There have been reports that up to 40,000 Russian troops, complete with heavy armour and fighter planes, have been massing on the Ukrainian border.

Russia issued a series of warnings to Ukraine that any action against people living within its borders would be regarded as a 'crime'.

Ukrainian forces, meanwhile, have been battling to wrestle back control of the occupied towns.

The Russian leader said: "We call on the representatives of southeastern Ukraine, the supporters of the federalisation of the country, to postpone the referendum planned for May 11.

"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 We have withdrawn them.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."

He made his comments after talks with the head of the Organisation for Security and Cooperation in Europe (OSCE), who said a "road map" would soon be put forward to defuse the Ukraine crisis.

The Pentagon and Nato both said they had no indication Russian military forces had withdrawn from the Ukraine border as yet.

Wednesday's announcement by Mr Putin was the first sign the Kremlin leader would not endorse the rebels' referendum planned for Sunday.

It was hailed as a breakthrough in what has been regarded as the worst crisis between East and West since the Cold War.

One of the separatist leaders said they would consider the Russian leader's call.

Denis Pushilin, in Donetsk, a city of one million people which rebels have proclaimed the capital of an independent 'People's Republic', said: "We have the utmost respect for President Putin.

"If he considers that necessary, we will of course discuss it."

Foreign Secretary William Hague, who is holding talks in Kiev with the country's leaders, told Sky News the rebels' referendum was an attempt to "disrupt" national elections that are being held in Ukraine on May 25.

Nato's top military commander said earlier it may deploy troops permanently in parts of Eastern Europe due to the increased tensions between Russia and Ukraine.

Meanwhile, Ukrainian forces withdrew from Mariupol city hall after a night of heavy fighting saw them recapture the rebel-held building.