WASHINGTON, June 16 (Reuters) - A duo of U.S. senators introduced a bill on Thursday to significantly enhance support for Taiwan, including provisions for billions of dollars in U.S. security assistance and changes to the decades old law undergirding Washington's unofficial ties with the Chinese-claimed democratic island.

The United States, which accuses China of ramping up military coercion toward Taiwan, is its main supporter and arms supplier, a point of increasing friction between Washington and Beijing, whose relations are already at their lowest point in decades.

The senators' Taiwan Policy Act of 2022 threatens severe sanctions against China for any aggression against Taiwan, and would provide $4.5 billion in foreign military financing over the next four years, as well as designate Taiwan a "major non-NATO ally," according to the text.

The sponsors, Senate Foreign Relations Committee chairman Bob Menendez, a Democrat, and Republican Lindsey Graham, said it would be the most comprehensive restructuring of U.S. policy toward Taiwan since the Taiwan Relations Act of 1979 - the bedrock of U.S. engagement with the island since Washington opened up relations with China that year.

"As Beijing continues to seek to coerce and isolate Taiwan there should be no doubt or ambiguity about the depth and strength of our determination to stand with the people of Taiwan and their democracy," Menendez said in a statement.

He said the bill sent a clear message that China should not make the same mistakes Russia made in invading Ukraine.

"The danger will only grow worse if we show weakness in the face of Chinese threats and aggression toward Taiwan," Graham said. Senate aides said the pair hoped to have the committee vote to send the bill to the Senate floor as early as next week.

Washington and Beijing have stood firm on their opposing views about Taiwan's right to rule itself.

China's embassy in Washington did not immediately respond to a request for comment on the proposed legislation.