Russia's President Vladimir Putin has called on pro-Russian separatists fighting government troops in eastern Ukraine to release Ukrainian soldiers they took prisoner and allow them to return home.

Putin said in a statement published on the Kremlin's web-site in the early hours on Friday that a lot of Ukrainian soldiers were fighting the insurgents "against them will" because they took an oath.

Ukraine on Thursday accused Russia of entering its territory with tanks, artillery and troops, and Western powers lashed at Moscow for lying about its role and dangerously escalated the conflict.

NATO said at least 1,000 Russian troops are in Ukraine and later released what it said were satellite photos of Russian self-propelled artillery units moving last week.