BRUSSELS, June 13 (Xinhua) -- NATO Secretary General Anders Fogh Rasmussen on Friday expressed concern over the "possible escalation" of the crisis in eastern Ukraine, according to a NATO statement.
Rasmussen said he was concerned about reports that "Russian tanks and other armored vehicles may have crossed the border into eastern Ukraine."
"If these reports are confirmed, this would mark a serious escalation of the crisis in eastern Ukraine," he said.
In Moscow, the Russian Foreign Ministry on Friday sent a note of protest to Ukraine over the border violation by Ukrainian soldiers.
"The state border was violated by armed units, as an infantry fighting vehicles crossed the border. An attempt was made to defy Russian border guards' orders," Kremlin spokesman Dmitry Peskov said on Friday.
An infantry fighting vehicle of Ukrainian armed forces crossed the Russian state border in the Rostov region of southwestern Russia earlier in the day, according to the Itar-Tass news agency.
Russian border guards detained the vehicle and its crewmen retreated. The later attempt to retrieve the vehicle was stopped by the guards. No arms were used and no casualties reported.