WASHINGTON - The United States signed a trade agreement Thursday with Taiwan over opposition from China, which claims the self-ruled island democracy as part of its territory.

The two governments say the U.S.-Taiwan Initiative on 21st Century Trade will strengthen commercial relations by improving customs, investment and other regulation.

The measure was signed by employees of the unofficial entities that maintain relations between the United States and Taiwan, a center for high-tech industry. They have no formal diplomatic ties but maintain unofficial relations and have billions of dollars in annual trade.

The agreement is intended to "strengthen and deepen the economic and trade relationship," the Office of the U.S. Trade Representative said in a statement. The deputy USTR, Sarah Bianchi, attended the signing.

The Chinese government accused Washington of violating agreements on Taiwan's status and demanded the U.S. government stop official contact with the island's elected government.

Taiwan and China split in 1949 after a civil war. The island never has been part of the People's Republic of China, but the mainland's ruling Communist Party says it is obligated to unite with China, by force if necessary.

Chinese leader Xi Jinping's government has stepped up efforts to intimidate Taiwan by flying fighter jets and bombers near the island. American and European politicians have visited Taiwan in a show of support for its elected government.

"The United States should stop any form of official exchanges with Taiwan' and 'refrain from sending wrong signals to the 'Taiwan independence' secessionist forces," said a Chinese foreign ministry spokesperson, Mao Ning.