Palau's president has accused China of flouting the Pacific nation's maritime borders, raising concerns about research vessels lurking "uninvited" within its waters.

"We keep on raising flags and complaining about it, but they keep on sending them," said President Surangel Whipps Jr., the pro-U.S. leader reelected this week.

"They continually don't respect our sovereignty and our boundaries and just continue to do these activities," he said from Palau's commercial center Koror.