WASHINGTON (AFP) - The United States said Monday it does not recognise the "illegal referendum" held over the weekend in east Ukraine that called for the region to break away from Kiev.

State Department spokesman Jen Psaki said the vote organised by pro-Russian elements in Donetsk and Lugansk "was an attempt to create further division and disorder in the country."

"We do not recognise the illegal referendum," she said. "It was illegal under Ukraine law."

Pro-Russian separatists claimed a massive victory in the two east Ukraine regions, while Kiev called the balloting a farce.

Psaki added that the "methodology was also highly suspect, with reports of ... pre-marked ballots, children voting, voting for people who were absent, and even voting in Moscow and St. Petersburg."

Some people had also voted more than once, Psaki alleged.

The United States and its allies remain focused on the presidential elections organised by the interim government in Kiev for May 25.

"They're very much on track with preparations for the elections," Psaki told reporters, adding that there were already a number of monitors on the ground.

She also denounced threats from the Russian company Gazprom to cut off gas supplies to Ukraine because of debts totalling US$3.51 billion (S$4.38 billion).

"Obviously, we don't think... access to energy should be used as a threatening tool," Psaki said.