US Secretary of State Hillary Clinton is missing the Democratic Party convention to build ties in Asia as the US relationship with China increasingly encroaches on politics back home.

Clinton, the most travelled Secretary of State in US history, was heading Sunday from a summit in the remote Cook Islands to Indonesia before she meets China's top leaders Tuesday and Wednesday in Beijing.

She will represent President Barack Obama at an Asia-Pacific summit in Vladivostok on September 8-9. The president formally launches his re-election bid at this week's Democratic convention in North Carolina.

Since narrowly losing the party nomination to Obama in 2008, Clinton has devoted her energy to foreign policy and carefully stayed out of domestic politics -- never showing an inch of public distance with her former rival while also maintaining cordial relationships with the rival Republican Party.

But China -- a major priority throughout her term -- has increasingly become a focus of Republican attacks with Obama's opponent Mitt Romney vowing a harder line on issues including trade, human rights and the military balance.

Representative Paul Ryan, Romney's pick for vice president, said that a Republican administration would do more to crack down on China's "cheating" on trade including "stealing" of US intellectual property.

Obama "said he'd go to the mat with China. Instead they're treating him like a doormat," Ryan told a rally in August in battleground state Ohio.

Romney has vowed that he would declare immediately that China is manipulating its currency by keeping the yuan artificially low and has accused Obama of keeping the US military "vastly under-resourced."

The Obama administration, while supporting some cuts in military spending, plans to reinforce the US presence in Asia and argues that it has filed a record number of trade complaints against China.

Clinton, asked in the Cook Islands about China's growing influence in the South Pacific, said that the United States wanted a "comprehensive, positive, cooperative relationship" between the world's two largest economies.

"We think it is good for our country, it's good for our people and, in fact, it's not only good for this region, it's good for the world," Clinton told reporters.

Clinton said that the United States also spoke "very frankly" on disagreements. She is expected in Asia to reaffirm calls for freedom of navigation in South China Sea, where Southeast Asian nations have accused Beijing of growing assertiveness.

China's state media have accused Clinton of seeking to "contain" the rising power through her trip, although Vice Foreign Minister Cui Tiankai sounded a more conciliatory note in the Cook Islands.

The state-run Xinhua news agency has had harsher words for Romney, accusing him of "poisoning the general atmosphere of US-China relations" through his "blame-China game."

Former US presidents Bill Clinton and George W. Bush also campaigned for a tougher line on Beijing -- Clinton's 1992 campaign spoke of the "butchers of Beijing" over the Tiananmen Square crackdown -- but chose cooperation once they entered the White House.

Still, Romney's promises are unusually specific. Nina Hachigian, a senior fellow at the Center for American Progress, a left-leaning think tank, said that branding China a currency manipulator would serve no purpose but potentially set off a trade war.

"It's not that we need to play 'nice' with China. We just need to find the best ways to further American interests. Basic human common sense suggests we should not anger China's top leaders and get nothing for it," Hachigian said.

Romney has also been strongly critical of Obama on Russia, calling Moscow the "number one geopolitical foe" of the United States.

Ernie Bower of the Center for Strategic and International Studies said he expected Clinton to "play it straight" on her trip and leave political squabbles up to Obama and his political team.

"Even for the Republicans, if you talk to the Romney people, they think that she has done a pretty good job," Bower said.

Clinton, who turns 65 next month, has repeatedly said that she will retire at the end of Obama's term in January regardless of the election and is not interested in another run for the White House.

But Clinton -- a hate figure for the right when she was first lady -- is now ranked in polls as one of the most popular US political figures, fuelling constant chatter of a new White House campaign in 2016.

Bill Clinton, who had an uneasy relationship with Obama in 2008 but has since rallied to his side, will deliver a key speech for him at the Democratic convention.