More than 400 Ukrainian soldiers have crossed the border into Russia and deserted the Kiev government, according to an Interfax news agency report quoting a border security official.
Ukraine confirmed the crossing, which happened on Monday, but said the soldiers were forced into Russian territory by rebel fire.
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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 opened a safe corridor for the soldiers into Russia. Late on Monday, Malayev said 180 of the troops had since been returned to Ukraine after consultations with Kiev while the rest were deciding what to do next.
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 would not say how many soldiers had crossed the border.
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 rocket attacks from separatist forces.
Rebel fighters fired for four hours from multiple heavy weapons, including tanks, mortars, artillery and Grad missile launchers, Dmitrashkovsky said, and eventually the brigade was forced to divide up into two sections to break out.
"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 on Monday, Interfax reported that Russia's air force began military drills in central and western regions of the country, a move that could stir further fears that Russia is ready to flex its military muscle in Ukraine.
The drills will start on Monday and last until Friday, Igor Klimov, air force chief, was reported as saying, and will involve more than 100 fighter jets and helicopters.
Also on Monday, Russia accused Kiev of deploying tactical missile launchers and multiple rocket-launch systems near Donetsk.
"The Ukrainian army keeps concentrating Tochka U rocket systems, as well as Smerch (Tornado) and Uragan (Hurricane multiple-launch rocket systems), near Donetsk," Russian Foreign Ministry said in a written statement.
Meanwhile, Kiev's inhabitants are bracing for chilly morning showers after hot water in the city was shut off on Monday until October in a bid to save gas following Russia's decision to stop supplies.
"All the thermal power plants have had their gas supply cut as of August 4," private firm Kievenergo, which is responsible for providing heating to Ukraine's capital city, said on Monday.
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 UN estimate.