(Adds no comment from U.S., Pelosi's office)

By Josh Smith

SEOUL, Sept 15 (Reuters) - China's top legislator, Li Zhanshu, is set to arrive in Seoul on Thursday and meet South Korean President Yoon Suk-yeol, who has sought to assure Beijing that his push to deepen ties with the United States is not a threat.

On Wednesday, Yoon's office said that it was seeking to arrange a meeting with Li for Friday, but that final details were being worked out. Li, chairman of the National People's Congress Standing Committee and currently ranked No.3 in the Chinese Communist Party, is also scheduled to meet the speaker of South Korea's national assembly.

Seoul is trying to manage its alliance with the United States and its relationship with China, South Korea's top trade partner, amid their intensifying rivalry.

Yoon has made strengthening ties with Washington a cornerstone of efforts to deter North Korea, but has insisted that none of its steps exclude or oppose Beijing.

Yoon's expected meeting with Li comes after the South Korean president was criticised, including by lawmakers in his ruling party, for not meeting U.S. House Speaker Nancy Pelosi during her trip last month that included a stop in self-ruled Taiwan to the fury of China.

Pelosi met the leaders of Singapore, Malaysia, Taiwan, and Japan during her trip.

Yoon, who was in Seoul but on vacation, had a phone call with her instead. His office said that the decision was made in consideration of national interests, and that there was no pressure from China, which welcomed the first high-level delegation from Yoon's administration days later.

Pelosi's office did not immediately respond to a request for comment. The White House declined to comment and the State Department referred queries to the South Korean government.

"I think the South Korean government is moving carefully to ensure that it ticks all of the appropriate protocol boxes with Li's visit, especially with China-South Korea relations at such a delicate stage," said Evans Revere, a former U.S. diplomat who has served in Seoul.

Yoon is making the effort to meet Li because of strong ties with the United States, not despite them, said Zhao Tong, a security scholar with Carnegie Endowment for International Peace.

"The Yoon administration prioritises substantive strategic cooperation with the United States but at the same time seeks an image of maintaining a balanced relationship between Washington and Beijing," he said. (Reporting by Josh Smith; Additional reporting by David Brunnstrom in Washington; Editing by Gerry Doyle and Raju Gopalakrishnan)