"I am concerned about reports that pro-Russian armed gangs are acquiring heavy weapons from Russia, including Russian tanks," Rasmussen said in a statement.
"We have seen reports that Russian tanks and other armoured 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 added.
Ukraine accused Russia on Thursday of allowing separatist rebels to bring three tanks and other military vehicles across the border into the east of the country to fight the Ukrainian army.