The United States said Monday it does not recognize the "illegal referendum" held over the weekend in eastern Ukraine that called for the region to break away from Kiev.

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

"We do not recognize 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 organized 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 $3.51 billion (2.55 billion euros).

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

And she welcomed a move by EU foreign ministers Monday to extend sanctions against Russians involved in the Ukraine crisis to 13 more people and two companies.

Washington has also expressed concerns to Paris over the $1.2 billion sale of two French Mistral warships to Russia -- an issue likely to be raised Tuesday when US Secretary of State John Kerry meets his French counterpart Laurent Fabius.

Fabius told CNN Monday that the order for the warships dated back to 2011 and "we have a rule when there is a contract it has to be implemented."

But he stressed that the first of the ships is not due for delivery in October. He reiterated that "the decision will be taken next October, but if we come to new sanctions it has to apply to defense, to finance as well, and to energy."

France would be on board with tougher sanctions against Russia over the Ukraine crisis "provided that everybody does those same sacrifices. It's not sanctions against Europe, it's sanctions against Russia," Fabius said.

He stressed that six members of the EU were totally dependent on Russian gas supplies while seven relied on Moscow for at least 50 percent of their gas needs.