Secretary of State Hillary Clinton said that the South Pacific was big enough for both the United States and China but urged the Asian power to ensure it distributes its growing aid fairly.
Clinton vowed that the United States would remain committed to the South Pacific "for the long haul" and offered new aid as she became the first US secretary of state to take part in an annual summit in the vast but sparsely populated region.
Her visit comes as several island states forge closer ties with China, which according to Australia's Lowy Institute has pledged more than $600 million in low-interest and mostly strings-free loans to the South Pacific since 2005.
Clinton, who will visit Beijing next week for talks on the often fractious relationship between the world's two largest economies, played down rivalries in the South Pacific during the summit in the tiny Cook Islands.
"We think it is important for the Pacific island nations to have good relationships with as many partners as possible and that includes China and the United States," Clinton told reporters.
Amid criticism that China's open wallet has undermined international pressure for democracy in Fiji and other nations, Clinton said: "Here in the Pacific, we want to see China act in a fair and transparent way".
Clinton, in an address to the 15-nation Pacific Islands Forum, said that all nations had "important contributions and stakes" in the security and prosperity of the region.
"I think, after all, the Pacific is big enough for all of us," she said, in a line she used repeatedly during her visit.
Chinese state media have accused Clinton of seeking to "contain" the rise of the Asian nation through her latest tour of the region.
But Vice Foreign Minister Cui Tiankai sounded a conciliatory note during the Pacific Islands Forum, saying Thursday that China was in "in this region not to seek any particular influence, still less dominance.
"We're here to be a good partner for the island countries, we're not here to compete with anyone," he told reporters.
Cui said that China was willing to work with other countries but added: "It will not mean that China will have to change its foreign aid policy. We are not changing it."
Any potential US attempt to contest China's role would also be fraught with difficulties as several nations in the region have embraced China, with Samoa's leader saying in June that the Asian power was a greater friend.
Prime Minister Henry Puna of the Cook Islands welcomed the renewed US interest in the region but made clear the region would not distance itself from China.
"We have a very close relationship with the People's Republic of China and I make no bones about it," he told reporters.
"They've been very good to us," Puna said. "There is certainly room for both in the Pacific."
Clinton said she spoke at length with New Zealand's Prime Minister John Key, who encouraged her trip to the summit, about the role of China.
Clinton said that the United States would welcome greater coordination with China on aid as well in protecting water resources and disaster relief.
Clinton announced $32 million in new aid projects, mostly to help Pacific islands plan ways to adapt to climate change -- a major concern for low-lying nations that fear being swamped by rising sea levels.
"We are increasing our investments," she said. "And we will be here with you for the long haul."
But the United States ended its main aid programmes in the South Pacific in 1994, resuming assistance only recently under President Barack Obama, leading some in the region to conclude that the United States was not interested.
The Obama administration has pledged a new focus on Asia, including shifting the bulk of the US Navy to the Pacific, as it sees a vital interest in a US role in shaping the future of the fast-growing and often turbulent region.
Clinton, during visits to China, Indonesia and Brunei, is expected to address rising tensions in the South China Sea where a number of Southeast Asian nations have accused Beijing of growing assertiveness.