U.S. Secretary of State John Kerry (L) shakes hands with Russian Foreign Minister Sergei Lavrov (R) at a bilateral meeting to discuss the ongoing situation in Ukraine as diplomats from the U.S., Ukraine, Russia and the European Union gather for discussions in Geneva April 17, 2014. REUTERS/Jim Bourg (SWITZERLAND - Tags: POLITICS)

Washington -

Russian troops massed on Ukraine's borders are moving back toward Moscow, but there are still “danger signs”, US Secretary of State John Kerry said late on Thursday.

“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.

He urged Russia to take advantage of the recent presidential elections and “build a road forward where Ukraine becomes a bridge between the West and the East”.

“The troops that were on the border are moving back towards Moscow not towards Kiev,” he said.

“There are still danger signs there that we hope will change.”

In a phone call on Wednesday, Russian Foreign Minister Sergei Lavrov had “expressed a hope that there might be a way forward”, Kerry said.

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 12 soldiers.

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 chocolate magnate Petro Poroshenko.

“We are disturbed by the ongoing violence in eastern Ukraine, including reports that separatists have shot down a Ukrainian military helicopter,” White House spokesman Jay Carney said.

“Now we cannot yet verify the details of these reports, but we are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside,” Carney added.

Moscow has consistently denied accusations of providing assistance to the pro-Russia separatists.

The Mi-8 helicopter gunship was shot out of the sky with a sophisticated surface-to-air missile. Among the dead was a Ukrainian general.

Acting President Oleksandr Turchynov said the weapon used was a Russian man-portable air defence system.

As well as hinting at Russian assistance, Carney voiced concern for a team of Organisation for Security and Cooperation in Europe observers being held by separatists in the city of Slavyansk, saying their detention was “unacceptable” and that they “should be released immediately”.

Separatists confirmed that they detained the four observers, who have been missing since Monday. - Sapa-AFP