Moscow - Russia on Monday announced large-scale air defence exercises along the border with Ukraine, further raising military tensions in the region.
More than 100 aircraft are taking part in the five-day exercises, between Monday and Friday, the Interfax news agency reported, citing Russian Air Force spokesperson Igor Klimov.
The report said that the exercises were unprecedented by ranging across three military districts. Fighter jets and attack helicopters would train by attacking targets in the air and on the ground, it said.
Russia has amassed troops and held exercises during the ongoing conflict between Ukrainian government forces and pro-Russian separatists in eastern Ukraine, while Nato has also held exercises in member countries bordering Ukraine.
Moscow also said on Monday that hundreds of Ukrainian soldiers had sought refuge in Russia, while Ukraine said that the servicemen had been cut off from their unit during fighting with separatists.
Some 438 soldiers asked Russian officials to grant them a "humanitarian corridor", Vasily Malayev, a spokesperson for Russian border guards in the region told Russian news agencies.
He said that the incident happened in the night between Sunday and Monday at the Gukovo border crossing between the Ukrainian Luhansk and the Russian Rostov region.
Russian Foreign Minister Sergei Lavrov told the Itar-Tass news agency that the soldiers' decision showed that the Ukrainian government's orders to fight their own people were "unacceptable".
Malayev later said that 180 of the Ukrainians were taken by bus back to Ukraine at their own request. He added that the remaining soldiers would remain in a camp in Russia's Rostov region.
A spokesperson for Ukraine's National Security and Defence Council, Andrei Lysenko, said that Ukrainian Foreign Ministry officials were in telephone contact with the soldiers.
"Negotiations are under way to return them to their homeland," Lysenko was quoted as saying by the Interfax Ukraine news agency. He did not say how many Ukrainian soldiers there were.
Much of the fighting in eastern Ukraine has been on the border with Russia, which Ukrainian government forces want to safeguard to prevent any influx of fighters and military hardware from Russia.