The US has accused Russia of using its energy supplies "as a tool of coercion" to try to control Ukraine.
It comes after Russia warned European countries of possible gas supply cuts because of Ukraine's energy debts.
Separately, Nato has defended the accuracy of satellite images which it says show Russian troops massed on the Ukrainian border in recent weeks.
A Russian official earlier said the images depicted a military exercise that was held in August last year.
Nato says the satellite images it unveiled early on Thursday show some 40,000 Russian troops near the border in late March and early April. They also reveal sophisticated warplanes and helicopters.
Later, Nato released additional images, some dating back to 2013 and others from early 2014, purporting to show the same areas unoccupied prior to March.
In a statement, the alliance says the images show that Russian claims that they are old were "categorically false" and that there was no evidence of military activity in these areas in 2013 or early 2014.
In Washington, state department spokeswoman Jen Psaki condemned "Russia's efforts to use energy as a tool of coercion".
She added that gas prices Ukraine was having to pay Moscow were not set by market forces, and that Washington was working with Ukraine to provide financing and help it find enough gas.
There are fears that current tensions between Russia and Ukraine could trigger gas shortages in Europe, as pipelines transiting Ukraine deliver Russian gas to several EU countries.