SYDNEY - The United States wants to expand security cooperation with the Pacific island nation of Papua New Guinea, a U.S. official said on Tuesday, amid concerns in Washington about China's motives for striking a security pact with the nearby Solomon Islands.

A U.S. delegation met PNG Prime Minister James Marape and its defence chiefs last week and plan to hold further security discussions in the coming months, said U.S. Assistant Secretary of State for East Asian and Pacific Affairs, Daniel Kritenbrink.

"There is a desire on both sides to ensure we take concrete steps to expand our security cooperation," he told reporters in a call.

On Friday, the United States said it had warned Solomon Islands Prime Minister Mannaseh Sogavare it would have "significant concerns and respond accordingly" to any steps to establish a permanent Chinese military presence.

China said previously the security agreement signed last week does not pose a risk to the United States. China has criticised Australia, which is less than 2,000 kilometres away and historically provided policing to the Solomon Islands, for opposing it.

Kritenbrink, part of a White House delegation that travelled to the Solomon Islands, Papua New Guinea and Fiji last week, said Sogavare had reiterated in the meeting that the China agreement was focused on domestic security needs and there would be no military base.

However the United States would monitor developments, because it was concerned by the lack of transparency and China's motives, he said.

"They are completely unclear because this agreement has not been scrutinised," Kritenbrink said, adding the concern was shared by regional partners, Pacific island nations and among Solomon Islands people.

The Solomon Islands occupies a strategic position for shipping lanes and communications in the Pacific and was the scene of some of the bloodiest fighting in that theater of World War Two, which the U.S. delegation highlighted in a visit to a war memorial.

China offered to redevelop a naval base in Papua New Guinea in 2018 but the government there struck a deal instead for Australia and the United States to upgrade a former World War Two U.S. naval base at Manus Island. Papua New Guinea is Australia's nearest northern neighbour.

"We do know that the PRC is seeking to establish a more robust overseas logistics and basing infrastructure that would allow the PLA to project and sustain military power at greater distances," Kritenbrink said, referring to the People's Republic of China and its military.

(Reporting by Kirsty Needham in Sydney; Editing by Jacqueline Wong)