Armored personnel carriers entered the eastern Ukrainian town of Slaviansk on Wednesday with the lead vehicle bearing the Russian flag, Reuters reported.
Troops inside the vehicles wore green camouflage uniforms and held automatic weapons and grenade launchers, according to the Associated Press, and at least one had the St. George ribbon attached to his uniform, which has become a symbol of the pro-Russian insurgency in eastern Ukraine.
One soldier, who identified himself only as Andrei, told the AP the unit was part of the 25th brigade of Ukraine's airborne forces and that they have switched to the side of the pro-Russian forces.
The troops stopped outside the town hall, which is occupied by separatists. The armed men waved as they drove in, and some people waved back and shouted "Russia! Russia!"
Overhead, a Ukrainian jet fighter carried out several minutes of aerobatics above the town's main square.
In the industrial city of Donetsk, at least 20 armed separatists occupied the city council building, a spokeswoman for the council said.
On Thursday the Russian and Ukrainian foreign ministers are due to meet in Geneva in the presence of the United States and the European Union, and the offensive came as Moscow warned Kyiv that tensions on the ground could escalate into civil war.
The arrival of the pro-Russian force in Slaviansk came less than a day after the delayed start to Ukraine’s “anti-terrorist operation” against pro-Russian rebels. On Tuesday, government forces fought off an attack Tuesday by about 30 gunmen at an airport.
Twenty-four hours after an ultimatum expired for armed men to lay down their guns, Ukrainian troops launched the strike against a group that had been in temporary control of the military landing strip in the eastern city of Kramotorsk, beginning what the president called an "anti-terrorist operation" to try to restore authority over the restive region.
The Kramatorsk airport has come under the increasing control of the pro-Russian gunmen who seized it last weekend.
The commander of the Ukrainian operation, Gen. Vasyl Krutov, speaking outside the Kramatorsk airport, said his men managed to thwart an attack by fighters who tried to storm the facility in the late afternoon. The attackers wore green military uniforms without insignia. An Associated Press reporter and camera crew heard rounds of gunfire at the time.
The mayor of Kramotorsk said that Ukrainian forces had taken back control of the military airport and were blocking its entrance.
Interim Ukrainian president Oleksandr Turchynov previously said the operation in the eastern Donetsk region would happen slowly, saying "the anti-terrorist operation began during the night in the north of Donetsk region. But it will take place in stages, responsibly, in a considered way."
Turchynov added that the aim of the operation was to "defend the citizens of Ukraine, to stop terror, stop crime and stop attempts to tear our country into pieces."
Russia cited the ongoing standoff as evidence that the country is falling further into instability.
"Blood has once again been spilt in Ukraine. The country is on the brink of civil war," Russian Prime Minister Dmitry Medvedev wrote on his Facebook page Tuesday, referring to at least two deaths on Sunday when Kyiv unsuccessfully tried to regain control in Slovyansk, one of about 10 towns and cities where the rebels have seized buildings.
Pro-Russian agitators appeared to dig in Tuesday. Despite Turchynov’s warning, many of the insurgents continued occupying government, police and other administrative buildings, demanding broader autonomy and closer ties with Russia.
Ukraine's security forces have been in some disarray since protesters ousted pro-Moscow President Viktor Yanukovych in February. However, the delays to the crackdown may also reflect a desire by the interim leadership to avoid making things worse by causing civilian casualties.
Kyiv’s ambassador to the United Nations in Geneva Yurii Klymenko said the unrest in the eastern part of the country was a “concocted pretext” engineered by Russia that aimed to disrupt Thursday’s four-way talks on Ukraine.
"This [pro-Russian] unrest was provoked by Russia. It's very remarkable that this unrest, these separatist actions, are taking place just before the talks. There is some linkage without any doubt," Klymenko said.
Klymenko also said that meeting with top U.S., European Union, Russian and Ukrainian diplomats would seek to reduce tensions on the Ukraine's borders with Russia, but added: "We are not going to discuss federalization of Ukraine."
Moscow accuses Kyiv of provoking the crisis by ignoring the rights of citizens who use Russian as their first language, and has promised to protect them from attack. It has also claimed the presence of far-right nationalists among Kyiv's new rulers.
However, a United Nations report on Tuesday 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 U.N. human rights office.
The report, issued after two visits to Ukraine last month by Assistant Secretary-General for Human Rights Ivan Simonovic, cited "misinformed reports" and "greatly exaggerated stories of harassment of ethnic Russians by Ukrainian nationalist extremists."
These, it said, "had been systematically used to create a climate of fear and insecurity that reflected on support to integration of Crimea into the Russian Federation."
Russian Foreign Minister Sergei Lavrov denied accusations from both Kyiv and the West that Russia was stirring up the separatists in the east and southeast as a possible prelude to repeating its annexation of Crimea.
"Ukraine is spreading lies that Russia is behind the actions in the southeast. This is a total lie that supposes that those residents there are completely incapable of protesting of their own will," Lavrov said on a visit to China.