John Kerry calls Russian foreign minister Sergei Lavrov, raises US concerns on Ukraine, Syria

United States Secretary of State John Kerry has raised concerns with Russia's foreign minister about reports of foreign fighters, including Chechens, crossing into Ukraine from Russia, the US State Department said on Thursday.

"He pressed Foreign Minister (Sergei) Lavrov to end all support for separatists, denounce their actions and call on them to lay down their arms," State Department spokeswoman Jen Psaki said of Kerry's telephone call with Lavrov on Wednesday.

The US secretary of state also urged Russia to engage with Petro Poroshenko, a confectionary magnate who on Sunday was elected Ukraine's president, the first Ukrainian since 1991 to win the presidency in a single round of voting. In the same call, Kerry also voiced concern about the delay in removing the remaining eight percent of chemical weapons materials from Syria as well as the recent detention of chemical weapons inspectors in Syria, Psaki said.

Pro-Russian separatists shot down a Ukrainian army helicopter on Thursday, killing 14 soldiers including a general, as government forces pressed ahead with an offensive to crush rebellions in the east swiftly following Poroshenko's election.

After weeks of accusations from Kiev of Russian involvement in the uprising, a rebel leader in the eastern city of Donetsk acknowledged that some of his fighters who died in the government offensive had been "volunteers" from Russia, saying their bodies were being returned across the border.