The Guardian reports,"Ukrainian troops were reported to be deployed outside a separatist-held city on Tuesday as the country's acting president claimed an "anti-terrorist operation" was under way".
An Associated Press reporter said at least 14 armoured personnel carriers with Ukrainian flags, one helicopter and military trucks were north of the eastern city of Slavyansk, where pro-Russian protesters took over key facilities on Saturday.
Government troops at a checkpoint outside the town of Izyum searched vehicles driving in and out for weapons.
Roads into Slavyansk were dotted with protester checkpoints, at least one with a Russian flag. Another bore a sign reading: "If we don't do it, nobody will."
The military moves came as western powers stepped up calls for Russiato act to end the crisis in Ukraine. The US president, Barack Obama, told his Russian counterpart, Vladimir Putin, in a tense telephone call on Monday night that Moscow should use its influence to get separatists in the country to stand down. The US has also deployed additional fighter aircraft to the Baltic states and Poland.
The British foreign secretary, William Hague, on Tuesday warned Russia it faced being frozen out by the west for years if it did not stop destabilising Ukraine, and the Nato chief, Anders Fogh Rasmussen, joined the US and UK in issuing an urgent public call on Russia to de-escalate tensions in the Ukraine.
Hague said it defied common sense for Putin to deny that the storming of government buildings in eastern Ukraine had been carried out by Russians.
The mayor of Slavyansk, where pro-Russian armed men have seized police and security service buildings, said on Ukrainian television on Tuesday that troops from Russia and Crimea had been involved in the takeovers.
After a mob seized police headquarters in Horlivka on Monday, a man in camouflage appointed a new police chief, later identifying himself as a lieutenant colonel in the Russian army.
Pro-Russian protesters, many of them armed, continued to occupy government, police and other administrative buildings in at least nine cities in the country's Russian-speaking east of the country, demanding broader autonomy and closer ties with Russia. The central government has so far been unable to rein in the separatists, and many of the local security forces have switched to their side.
Early on Tuesday, pro-Russian separatists pulled out of one building they had occupied in Kramatorsk but then seized another in the same town, officials said.
Obama told Putin on Monday that the US preferred a diplomatic solution to the crisis but criticised Russia for taking actions that were not "conducive" to such a path. He said Kiev had made "real offers" to address concerns about the decentralisation of powers to local governments in the country, a senior US government official said.
In a sign of rising tensions, the Pentagon said on Monday that a Russian military fighter flew past a navy destroyer in the Black Sea 12 times over the weekend in a "provocative and unprofessional" move.
The unarmed fighter Su-24 aircraft, or Fencer, made 12 low-altitude passes of the USS Donald Cook on Saturday, coming within 1,000 yards of the vessel, according to the Pentagon. The incident lasted around 90 minutes and a Pentagon source said the USS Donald Cook "was never under threat" and was "more than capable of defending herself".
