Brussels: Russia has increased its presence of troops on the Ukraine border to 20,000, from 12,000 in mid-July, creating a "dangerous situation" and stoking concerns Moscow could intervene in its neighbour by force, NATO said on Wednesday.
NATO spokeswoman Oana Lungescu said recent Russian actions had escalated the crisis and undermined efforts to find a diplomatic solution.
"Russia has amassed around 20,000 combat-ready troops (on the border).... This is a dangerous situation," Lungescu said.
"We share the concern that Russia could use the pretext of a humanitarian or peacekeeping mission as an excuse to send troops into eastern Ukraine," she said, charging that Moscow continued to support pro-Russian rebels in Ukraine and allow weapons to cross the border.
"Any deterioration of the humanitarian situation in separatist-held areas... Is due to Russia`s continued destabilisation of Ukraine," she said.
Lungescu reiterated that Russia had to halt all such support and withdraw "all its military forces on the border".
Earlier this year, after Russia annexed Ukraine`s Crimea peninsula, NATO put Russian troop strength at 40,000 but then steadily reduced the numbers to less than 1,000 by June as Moscow appeared ready to explore diplomatic initiatives to solve the crisis.
There was a subsequent increase in Russian troops and NATO`s top commander, General Philip Breedlove, said last week that Russia had increased numbers to "well over 12,000".
Ukrainian government forces have been making solid gains in recent weeks against pro-Russian rebels and now have the main separatist stronghold of Donetsk in their sights.
The UN says the fighting has forced some 285,000 people to flee their homes, with Kiev and Moscow blaming each other for the exodus.
Russia`s ambassador to the UN Vitaly Churkin said yesterday the situation in east Ukraine was "disastrous" and called on the international community "to take emergency measures to improve the humanitarian situation" there.
"When addressing the humanitarian situation, we cannot lose sight of one underlying fact: Russia can stop all of this," US Deputy Ambassador Rosemary DiCarlo retorted.
"The surest way to end the violence is for Russia to stop the flow of fighters, weapons and money from Russia into eastern Ukraine," DiCarlo said.