WASHINGTON, July 28 (Reuters) - U.S. President Joe Biden and China's Xi Jinping may hold their fifth call as leaders as soon as today, as concerns rise over a possible visit to Chinese-claimed Taiwan by U.S. House Speaker Nancy Pelosi.

White House officials have said the long-planned call will have a broad agenda, including discussion of Russia's invasion of Ukraine, which China has yet to condemn.

At its core, U.S. officials see the exchange as another chance to manage competition between the world's two largest economies, whose ties are increasingly clouded by tensions over democratically governed Taiwan, which Xi has vowed to reunite with the mainland, by force if necessary.

Beijing has issued escalating warnings about repercussions should Pelosi visit Taiwan, a move that would be a dramatic, though not unprecedented, show of U.S. support for the island, which says it is facing increasing Chinese military and economic threats.

Washington does not have official relations with Taiwan and follows a "one-China" policy that recognizes Beijing, not Taipei diplomatically. But it is obliged by U.S. law to provide the island with the means to defend itself, and pressure has been mounting in Congress for more explicit support.

"This is about keeping the lines of communication open with the president of China, one of the most consequential bilateral relationships that we have, not just in that region, but around the world, because it touches so much," White House national security spokesman John Kirby told reporters on Wednesday.

One person briefed on planning for the call said the Biden administration thinks leader-to-leader engagement is the best way to lower tensions over Taiwan.

Xi has an interest in avoiding a tense confrontation with the United States as he seeks an unprecedented third term in office at a congress of China's ruling Communist Party, which is expected in October or November, some analysts believe.

Biden also wants to discuss climate and economic competition issues, the person briefed said, as well as the idea of placing a price cap on Russian oil to punish Moscow for its war in Ukraine, an issue Treasury Secretary Janet Yellen raised with Chinese counterparts earlier in July.

The Biden administration has been debating whether to lift some tariffs on Chinese goods as a way to ease soaring inflation, but U.S. officials have said a decision was not expected ahead of the call.

When Biden last spoke to Xi in March, he warned of "consequences" if Beijing gave material support for Russia's war, and the U.S. government believes that that red line has not been crossed in the months since.