The Kremlin said on Wednesday that Russia would keep military forces near its border with Ukraine for as long as it saw fit and a high-ranking Moscow security official added it could take "measures" if necessary.
Western nations have called for restraint after Ukraine raised the alarm over a buildup of Russian forces near its border and violence rose along the line between Kyiv's troops and separatists in Ukraine's east.
Russia's Security Council secretary, Nikolai Patrushev, said Russia had no plans to intervene in the conflict.
"We don't have such plans, no. However, we are closely watching the situation. Concrete measures will be taken depending on how it develops," Patrushev said in an interview with the Kommersant newspaper.
Kyiv on Tuesday called on NATO to lay out a path for it to join the Western military alliance, drawing an immediate rebuke from Moscow, which fiercely opposes the bloc's eastern expansion.
Oleksiy Danilov, secretary of Ukraine's national security and defence council, told Reuters that Kyiv believes Russia is massing troops and military hardware on the border to try to rally Russians against a foreign enemy and distract from domestic problems.
A meeting of a trilateral working group on Ukraine's rebel-held Donbass region ended on Wednesday without any tangible results, said Russia's envoy to the group, Boris Gryzlov. Gryzlov said he welcomed Ukraine's pledge to stick to a ceasefire in the Donbass.
The Russian foreign ministry's spokeswoman accused Kyiv of bolstering its military forces and hardware at the contact line with separatists in the Donbass.
Russia has said its military movements near its border with Ukraine are defensive and pose no threat.
Earlier on Wednesday, Kremlin spokesman Dmitry Peskov was asked how long Russian forces would be stationed near Ukraine.
"Russia's armed forces are on Russian territory in the places it considers necessary and appropriate, and they will stay there for as long as our military leadership and supreme commander consider it appropriate," Peskov said.
SITUATION IS WORSENING
Russian-backed separatists have fought since 2014 against Ukrainian forces in the Donbass, a conflict that Kyiv says has killed 14,000 people.
"The situation on the line of contact remains, alas, extremely unpeaceful, the situation is escalating and changing for the worse. The amount of shelling is increasing," Denis Pushilin, a separatist leader in Donbass, was quoted as saying by TASS news agency.
Ukraine and Western countries say Donbass separatists have been armed, led, funded and aided by Russians, including active Russian troops. Moscow has denied interfering. While a ceasefire halted full-scale warfare in 2015, sporadic fighting never ceased.