By Roberta Rampton and Timothy Gardner

WASHINGTON (Reuters) - President Donald Trump will use fast-growing supplies of U.S. natural gas as a political tool when he meets in Warsaw on Thursday with leaders of a dozen countries that are captive to Russia for their energy needs.

In recent years, Moscow has cut off gas shipments during pricing disputes with neighboring countries in winter months. Exports from the United States would help reduce their dependence on Russia.

Trump will tell the group that Washington wants to help allies by making it as easy as possible for U.S. companies to ship more liquefied natural gas (LNG) to central and eastern Europe, the White House said.

Trump will attend the "Three Seas" summit - so named because several of its members surround the Adriatic, Baltic and Black Seas - before the Group of 20 leading economies meet in Germany, where he is slated to meet Russian President Vladimir Putin for the first time.

Among the aims of the Three Seas project is to expand regional energy infrastructure, including LNG import terminals and gas pipelines. Members of the initiative include Poland, Austria, Hungary and Russia's neighbors Latvia and Estonia.

Trump's presence will give the project a lift, said James Jones, a former NATO Supreme Allied Commander.

Increased U.S. gas exports to the region would help weaken the impact of Russia using energy as a weapon or bargaining chip, said Jones.

"I think the United States can show itself as a benevolent country by exporting energy and by helping countries that don’t have adequate supplies become more self-sufficient and less dependent and less threatened," he said.

Trump's Russia policy is still taking shape, a process made awkward by investigations into intelligence findings that Russia tried to meddle in the 2016 U.S. presidential race. Russia denies the allegations and Trump says his team did not collude with Moscow.

Lawmakers in Trump's Republican Party, many of whom want to see him take a hard line on Russia because of its interference in the election and in crises in Ukraine and Syria, support using gas exports for political leverage.

"It undermines the strategies of Putin and other strong men who are trying to use the light switch as an element of strategic offense," said Senator Cory Gardner, a Republican from Colorado who is on the Senate Foreign Relations Committee.

The Kremlin relies on oil and gas revenue to finance the state budget, so taking market share would hurt Moscow.

"In many ways, the LNG exports by the U.S. is the most threatening U.S. policy to Russia," said Michal Baranowski, director of the Warsaw office of think-tank the German Marshall Fund.