Kiev: Russian troops massed on Ukraine's borders are moving back toward Moscow, but there are still "danger signs", US Secretary of State John Kerry says.

"There is evidence of Russians crossing over, trained personnel from Chechnya trained in Russia, who've come across to stir things up, to engage in fighting," the top US diplomat told PBS television.

Separatist fighters tear apart a Ukrainian flag outside the regional government headquarters in Donetsk, eastern Ukraine, on Thursday. Credit:AFP

The White House, meanwhile, expressed concern that pro-Russian separatists in Ukraine were using advanced weapons "from the outside" after they shot down an army helicopter, killing 14 soldiers including a general.

The downing of the aircraft came amid escalating clashes between Kiev's forces and separatists in the eastern part of the country, following the weekend election win of oligarch Petro Poroshenko.