A Russian border security official said more than 400 Ukrainian soldiers crossed into Russia, according to a report from the Interfax news agency. It wasn't immediately clear why the soldiers entered Russia, with both sides giving conflicting accounts.

The Russian official said the soldiers deserted the Kiev government and the Russian side opened a safe corridor, while a Ukrainian military official said the soldiers, without giving a number, were forced into Russian territory by rebel fire after running out of ammunition.

Vasily Malayev, head of the Federal Security Service's border patrol in the Rostov region, told Interfax that 438 Ukrainian soldiers had reached Russia on Monday. He said the Russian side had allowed the soldiers to safely enter the country.

Russia's Defense Ministry couldn't immediately be reached for comment.

The Ukrainian military confirmed that part of a brigade had most likely crossed into Russian territory, although it disputed Russia's version of events and wouldn't say how many soldiers went over.

A spokesman for the Ukrainian military operation in the east, Oleksiy Dmitrashkovsky, said troops from the army's 72nd brigade were penned into their position and came under a sustained barrage of fire from separatist forces. Rebel fighters used tanks, mortars, artillery and Grad missile launchers over four hours, Dmitrashkovsky said, and eventually the brigade was forced to divide up into two sections.

"One was meant to break out and join forces with a support unit. The other unit had the task of providing fire cover," Dmitrashkovsky said. "In doing that they fired their weapons until no ammunition remained, after which they abandoned their position and reached a place near a border crossing on Russian territory."

Dmitrashkovsky said it was too early to confirm how many soldiers had crossed into Russia.

"We do not have such information. The Russians are capable of claiming anything they want," he said.

Earlier Monday, Interfax reported that Russia's air force began military drills in central and western regions of the country, a move that could spark further fears that Moscow is ready to flex its military muscle in Ukraine.

The drills will start Monday and last through Friday, air force chief Igor Klimov was reported as saying, and will involve more than 100 fighter jets and helicopters.

Pro-Russian separatists in eastern Ukraine have been battling the Kiev government since April, a conflict that has claimed at least 1,129 civilian casualties, according to a U.N. estimate. Ukraine and Western countries have accused Russia of providing the rebels with equipment and expertise, a claim that the Russian government has repeatedly denied.