Observers from the Organization for Security and Cooperation in Europe (OSCE) have left the Gukovo border checkpoint in southwest Russia over security concerns, Russian authorities said Monday.

"The OSCE officials have left the border checkpoint due to the shooting and explosions of munitions, whose fragments are reaching the Russian territory from time to time," said Vasily Malayev, a spokesman for the Russian Federal Security Service's (FSB) Rostov regional department,

He added that Russian border guards and customs officers remained on duty.

The OSCE Permanent Council decided to send an observer mission to the Russian-Ukrainian border on July 24, under the Berlin Declaration signed by the foreign ministers of Germany, Russia, France and Ukraine on July 2.

The first group of OSCE officials, composed of four observers and three staff members, began monitoring the checkpoints of Gukovo and Donetsk on July 30.

Meanwhile, it was reported that the checkpoints came under fire several times, with no proof of who should be held accountable.

Earlier Monday, the FSB said more than 400 Ukrainian servicemen had sought refuge in Russia, without elaborating the reasons.

"Border guards opened a humanitarian corridor and let 438 Ukrainian troops into Russia," Interfax news agency quoted Malayev as saying, who added that among the Ukrainians soldiers were 160 border guards.

Until around 2:30 p.m. Moscow time (1030 GMT), 180 of the soldiers have returned to Ukraine by buses, said the spokesman.

As clashes kept raging in eastern Ukraine, Russia has been trading blames with major Western powers over build-up of military presence near its borders with Ukraine.