Ukraine's president-elect, Petro Poroshenko, has promised to punish pro-Russian rebels who shot down an army helicopter in the east of the country, killing 14 soldiers, including a general.
The country's newly elected leader called the separatists, who brought down the helicopter gunship in Slovyansk, "terrorists" and "bandits".
The fighters shot down the Mi-8 helicopter with a sophisticated surface-to-air missile on Thursday, prompting the US to say the incident raised concerns about rebels being supplied "from the outside".
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The White House has previously expressed concern at Russia's alleged role in supporting rebels in Ukraine's eastern regions, and has asked Moscow to use its influence to end the conflict.
The Russian president, Vladimir Putin, rejected accusations that his country supported the rebels but a separatist leader said on Thursday that 33 out of the 40 rebels killed in a battle for Donetsk airport were Russian nationals from Muslim regions, such as Chechnya.
The revelation supports Kiev's claims that the rebels do not represent the true will of the miners and steel workers who have turned the east into the economic engine of Ukraine.
The US secretary of state, John Kerry, said there was evidence of "personnel from Chechnya trained in Russia" crossing in to Ukraine to "stir things up", the AFP news agency reported.
Kerry urged Russia to take advantage of Sunday's presidential election and "build a road forward where Ukraine becomes a bridge between the West and the East".
Russian troops were also moving away from Ukraine's borders, but there are still "danger signs", he told PBS television.
Moscow, meanwhile, called on Kiev to impose an immediate ceasefire and urged the West to use its influence to prevent "a national disaster" in Ukraine.
The Russian foreign ministry said peace would be impossible without the withdrawal of Ukrainian troops from the country's eastern provinces.
Rebels seized parts of eastern Ukraine in response to the February removal in Kiev of pro-Kremlin president Viktor Yanukovich.
Many want to follow the example of Crimea, which broke away from Ukraine and was annexed by Russia, following a referendum in March.
Western countries, which have supported Ukraine's new government, have responded to Moscow's takeover of Crimea with sanctions against officials and organisations close to Putin.