WASHINGTON (Reuters) -U.S. President Joe Biden will host a second summit with leaders of Pacific island nations in September, Joseph Yun, the U.S. official responsible to negotiations with three key Pacific island states said on Thursday.

U.S. officials have said the summit is expected to take place around the time of the U.N. General Assembly meetings in New York in September.

Biden's national security adviser, Jake Sullivan, said in May Biden would arrange another summit of Pacific island leaders this year after the disappointment caused by his cancellation of a visit to Papua New Guinea due to the domestic debt ceiling crisis. Biden hosted a first summit with the leaders last year.

"He has committed (to) holding a second summit in September this year," Yun told an event hosted by the Heritage Foundation think tank.

Yun stressed the importance of the U.S. Congress approving new funding for the three Pacific Island countries he has been negotiating with by the end of the fiscal year on Sept. 30, given China's strategic ambitions in the region.

Palau, Micronesia and the Marshall Islands all have what are known as Compacts of Free Association (COFAs) with the United States, under which the U.S. has responsibility for their defense and provides economic assistance, while gaining exclusive access to huge strategic swathes of the Pacific.

In May, Yun said he had finalized terms with Micronesia and Palau to renew their COFAs and hoped do so with the Marshall Islands in coming weeks.

However, the foreign minister of the Marshall Islands called last week for more U.S. money to deal with the legacy of massive U.S. nuclear testing to enable the renewal of its agreement, the economic terms of which expire on Sept. 30.

"I'm very hopeful that we can reach an agreement in a reasonably short time - weeks," Yun said.

Under MOUs agreed this year, the U.S. will commit a total of $7.1 billion over 20 years to the three nations, subject to congressional approval.

Yun has proposed that Congress approve the total amount agreed in the MOUs by Sept. 30, even without a final agreement with the Marshall Islands, warning that it would be damaging to U.S. credibility and a big blow for the islands if that did not happen.

(Reporting by David Brunnstrom and Trevor Hunnicutt; Editing by Stephen Coates)