NATO sources have confirmed the Russian military invasion of Ukraine after a column of 32 tanks entered the country from Russia.

Ukraine's military spokesman Andriy Lysenko said a convoy made up of 32 tanks, 16 howitzer cannons and 30 trucks of troops and equipment had crossed the border into the rebel-controlled Luhansk region.

"The deployment continues of military equipment and Russian mercenaries to the front lines," he said in a televised briefing.

The move comes the day after fighting with pro-Russian rebels killed five Ukrainian soldiers and injured 16, he added.

A Kremlin advisor said that Russia was committed to the two-month-old Minsk agreement on ceasefire in eastern Ukraine and wanted to build on peace talks.

Foreign policy adviser Yuri Ushakov said Russia respected the will of voters after a leadership election in eastern Ukraine, seen as illegitimate in the west.

But he was careful to add that he had deliberately chosen the word "respect" rather than "recognise".

Russian president Vladimir Putin is due to meet with a number of world leaders next week, Mr Ushakov said.

The conflict and EU and US sanctions have hit Russia hard, with the ruble set for its worst week in 15 years.

So far this year it has lost more than a third of its value against the US dollar.