European mediators in the Ukrainian crisis say two men captured by Kyiv's troops had confessed to being members of the Russian armed forces sent in to back up pro-Moscow separatist fighters.
The revelation by the Organization for Security and Cooperation in Europe (OSCE) provides some of the strongest independent evidence to date of Russian President Vladimir Putin's direct involvement in the 13-month war in the neighboring nation.
Kyiv and its Western allies have long accused Russia of backing up rebels with weapons and troops.
Russia denies the allegations.
The OSCE said it interviewed the two men in a military hospital on May 20, after they were wounded and taken prisoner by Ukrainian government forces last week near a part of the eastern province of Luhansk controlled by separatists.
The OSCE said in a report, "Both individuals claimed that they were members of a unit of the armed forces of the Russian Federation. They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack."
"One of them said he had received orders from his military unit to go to Ukraine; he was to 'rotate' after three months. Both of them said they had been to Ukraine 'on missions' before," the OSCE added.