Ukraine Prime Minister Arseniy Yatsenyuk has claimed that Russia has sent military forces and equipment to support pro-Russian rebels in eastern Ukraine.
Yatsenyuk was quoted by Ukraine's state-run media as saying: "Ukrainian military intelligence confirms the fact military personnel and equipment have been transferred from Russia to Ukraine.
"Tanks, GRAD multiple rocket systems, BUK and SMERCH systems, [as well as] radio electronic intelligence systems, are not sold at local Donetsk street markets.
"Only the Russian Army and defence ministry have them."
Yatsenyuk's spokesperson Olga Lappo confirmed to CNN that the quotes attributed to the prime minister are accurate.
In addition, the Ukrainian National Security and Defence Council has claimed in a statement seen by The New York Times that approximately 700 soldiers crossed Russia's western border into eastern Ukraine.
Russian officials could not be immediately reached for comment.
The country has repeatedly denied helping the rebels with the supply of weaponry and the presence of its regular forces in eastern Ukraine.
Russia has, however, admitted that several 'volunteers' are fighting alongside the separatists, BBC News reported.
The accusation comes days after Ukrainian Security Forces reportedly recaptured all lost ground around Donetsk airport after fierce fighting that killed three soldiers and left 66 injured.
Russia has expressed concern at what has dubbed as escalation by the Ukrainian military.
Russian Deputy Foreign Minister Grigory Karasin was quoted by Interfax as saying: "It's the biggest, even strategic, mistake of the Ukrainian authorities to bank on a military solution to the crisis.
"This may lead to irreversible consequences for Ukrainian statehood."
Ukrainian military forces and rebels are fighting for control of major cities across the eastern and southern regions of Ukraine.
Russia annexed the Crimean Peninsula in March 2014.
According to a UN estimate, more than 4,700 people have been killed in the fighting.
Image: The main terminal of the Donetsk International Airport prior to fighting between separatists and Ukrainian Security Forces. Photo: courtesy of Michael1238.