WASHINGTON (Reuters) – The United States sent two Navy ships through the Taiwan Strait on Monday as the U.S. military increased the frequency of movement through the strategic waterway despite opposition from China.

The U.S. Navy’s passage through the Taiwan Strait also comes just days before a summit between Trump and North Korea’s Kim Jong Un.

“The ships’ transit through the Taiwan Strait demonstrates the U.S. commitment to a free and open Indo-Pacific,” the U.S. Pacific Fleet said in a statement.

The two ships were identified as the destroyer Stethem and Navy cargo and ammunition ship Cesar Chavez, the statement added. The 180-km-wide (111.85 miles) Taiwan Strait separates Taiwan from China.

Washington has no formal ties with Taiwan, but is bound by law to help defend the island nation and is its main source of arms. The Pentagon says Washington has sold Taiwan more than $15 billion in weaponry since 2010.

China has been ramping up pressure to assert its sovereignty over the island, which it considers a breakaway province of “one China.”

China has repeatedly sent military aircraft and ships to circle the island on drills in the past few years and worked to isolate the island internationally, whittling down its few remaining diplomatic allies.

Earlier this year the U.S. Defense Intelligence Agency released a report describing Taiwan as the “primary driver” for China’s military modernization, which it said had made major advances in recent years.

Taiwan is one of a growing number of flashpoints in the U.S.-China relationship, which also include a trade war, U.S. sanctions and China’s increasingly muscular military posture in the South China Sea, where the United States also conducts freedom of navigation patrols.