Russian President Vladimir Putin has warned Ukraine is "on the verge of civil war."
The Kremlin said Putin made the comment in a telephone call with German Chancellor Angela Merkel on the situation there.
Putin spoke hours after Ukrainian armed forces retook control of a small military air base in the east of the country, where pro-Russian separatists have seized control of buildings and other facilities in at least nine cities.
Acting President Oleksandr Turchynov said the base in Kramatorsk had been "liberated," as an antiterrorist operation finally got under way.
There were conflicting reports on casualties. Russian media said at least four people had been killed in Kramatorsk. Kyiv said there were no casualties, but added four armed pro-Russian separatists had been detained.
Another "antiterrorist" operation was also reportedly under way in nearby Slovyansk, although few details have emerged there.
The White House described Ukraine's response to an "untenable" situation in the east as "measured."
Earlier, Putin called for the United Nations and international community to condemn the Ukrainian operation in a phone call with UN Secretary-General Ban Ki-moon.
Moscow has blamed the government in Kyiv for stirring up problems by ignoring the rights of citizens who use Russian as their first language, and has promised to protect them from attack.
However, a United Nations report on April 15 cast doubt on whether Russian-speakers were seriously threatened, including those in Crimea who voted to join Russia after Moscow forces had already seized control of the Black Sea peninsula.
"Although there were some attacks against the ethnic Russian community, these were neither systematic nor widespread," said the report by the UN human rights office.
Russia called the report one-sided, politicized and apparently fabricated.
NATO Secretary-General Anders Fogh Rasmussen accused Moscow of involvement in the rebellions in Ukraine.
"It is very clear that Russia's hand is deeply engaged in this," he told reporters at a meeting of EU defense ministers in Luxembourg.
Russian Foreign Minister Sergei Lavrov denied that Moscow was agitating the separatists in the east and southeast.
But Ukrainian First Deputy Prime Minister Vitaly Yarema accused Russia of sending its troops to Ukraine, saying Kyiv has "evidence that those people occupying Slovyansk and Kramatorsk right now are servicemen of the Russian 45th Airborne Regiment."
Ukraine's state Security Service (SBU) also identified one of the leaders of the pro-Russian operation in Slovyansk as a Russian foreign intelligence agent named Igor Strelkov.
The SBU said Strelkov also coordinated Russian seizures of military facilities in Crimea.
In an interview with RFE/RL, Ukraine’s former Prime Minister Yulia Timoshenko, who is running for president, called for the creation of national brigades to defend the country against what she describes as a "war" launched by Russia aimed at seizing Ukrainian territory.
With reporting by AP and Reuters