Ukraine has claimed that Russia has 45,000 troops with tanks, missile systems, war planes and attack helicopters on its border.
"As of 11 o'clock today, about 45,000 troops of the armed forces and internal forces of the Russian Federation are concentrated in border areas," said military spokesperson Andriy Lysenko.
He said they were supported by 160 tanks, 1,360 armoured vehicles, 390 artillery systems, up to 150 Grad missile launchers, 192 fighter aircraft and 137 attack helicopters.
Meanwhile, European Commission President José Manuel Barroso has warned Russian President Vladimir Putin against unilateral military actions in Ukraine.
"President Barroso warned against any unilateral military actions in Ukraine, under any pretext, including humanitarian," the Commission said in a statement.
Mr Putin told Mr Barroso in the same phone conversation that Russia is coordinating with the International Red Cross to send humanitarian aid to Ukraine.
The Red Cross has said it is ready to help with the handover of aid but it needs agreement and security guarantees from all sides.
US President Barack Obama told his Ukrainian counterpart Petro Poroshenko that any Russian intervention in Ukraine without consent from the Ukrainian government would be "unacceptable" and a violation of international law, the White House said.
In a phone call Mr Poroshenko told Mr Obama that there has been continued shelling of Ukrainian territory from Russia, the White House said.
Mr Obama urged Mr Poroshenko to exercise restraint in military operations in order to avoid civilian casualties, it added.
NATO chief Anders Fogh Rasmussen said he saw a "high probability" that Russia could intervene militarily in eastern Ukraine.
He said NATO saw no sign that Russia was pulling back its forces from close to the Ukrainian border.
Asked how high he rated the chances of a Russian military intervention, Mr Rasmussen said: "There is a high probability.
"We see the Russians developing the narrative and the pretext for such an operation under the guise of a humanitarian operation and we see a military build up that could be used to conduct such illegal military operations in Ukraine."
Meanwhile, more than 100 inmates broke out of jail when shells hit a high-security prison in the besieged rebel stronghold of Donetsk in eastern Ukraine, killing one, local authorities said this morning.
Mortar blasts rocked the facility in a western district of the city last night, hitting the living areas, administrative headquarters and an electrical substation, the city council said in a statement.
The bombardment sparked a mass jail break that saw scores of detainees flee the prison.
"A riot started in the facility and 106 people escaped their place of detention," the council said, adding that three convicts were seriously wounded by the shelling.
By this morning some had been returned to the facility, with a prison official telling AFP that 40 inmates were still missing and thought to be hiding in buildings around the prison.
An AFP correspondent at the scene found the prison gates open and rebel gunmen patrolling around.
A shell hole 50cm deep could be seen in the asphalt in a jail yard.
A spokesman from the rebel Vostok battalion going by name Koba said that insurgent fighters had come to secure the location over fears that escaped prisoners could try to get arms.
Inner city Donetsk has been pounded by heavy shelling over the past few days as Ukrainian forces have surrounded the city and vowed to retake the pro-Russian rebel stronghold.
A growing number of civilian casualties have been reported as artillery bombardments have hit hospitals and homes around the beleaguered city.
More than 1,300 have been killed and more than 285,000 people have fled their homes in the east due to fierce clashes in four months of what the Red Cross has already deemed a civil war.