South Korea's ruling party chief has said the United States is more important to the country than China, stressing Washington is the "irreplaceable" ally for Seoul.

"I think we should not forget that the U.S. is the irreplaceable, unequaled and only ally for us," Kim said during a meeting with South Korean correspondents in Washington on Monday.

"So I came here for parliamentary diplomacy aimed at reaffirming the alliance through meetings with congressional leaders and government officials."

Kim has been on a nine-day visit to the U.S. that includes a series of meetings with U.S. government officials and congressional leaders, including Deputy Secretary of Defense Robert Work and Sen.

John McCain, chairman of the Senate Armed Forces Committee.

On Tuesday, Kim is also scheduled to hold a series of meetings, including one with Secretary of State John Kerry.

"When I meet them, I will talk about the importance of the friendship and alliance between South Korea and the U.S. In particular, I will make clear that for us, it is the United States, rather than China (that is more important to the country)."

The U.S. has long been the biggest ally for South Korea since they fought to repel invading North Korean troops backed by China.

About 28,500 American troops are stationed across South Korea to help defend the Asian ally.

But Seoul's economic ties to Beijing have grown stronger since they established diplomatic relations in 1992, and China has overtaken the U.S. as South Korea's No. 1 trade partner. Their political and security relations have also moved forward, though not as fast as economic ties.

Since taking office in early 2013, South Korean President Park Geun-hye has also tried to shore up relations with China amid widespread perceptions that her predecessor, Lee Myung-bak, put too much emphasis on strengthening the alliance with the U.S.

Chinese President Xi Jinping is the first Chinese leader to visit South Korea before North Korea.

South Korean and U.S. officials have said that Seoul's relations with Washington and Beijing are not a "zero-sum game."

American officials have said in particular that the U.S. encourages Seoul to have better relations with China. (Yonhap)