US President Donald Trump said on Saturday that China was helping to rein in North Korea, but he hoped Beijing would do more.
Speaking to his Vietnamese counterpart, Tran Dai Quang in Hanoi, Trump said he could employ his diplomatic skills to help Asian nations come to an agreement over competing maritime claims.
"It is a very kind, generous offer because he is a good mediator".
In a joint news conference, Quang said he had "shared my thoughts with (Trump) on the recent developments in this area and it is our policy to settle disputes in the South China Sea through peaceful negotiations, and with respect for diplomatic and legal process in accordance with worldwide law, including the 1982 UN Convention on the Law of the Sea".
Despite unresolved issues, discussions between ASEAN and China have improved, Cayetano said.
Mr Trump's offer comes ahead of Chinese president Xi Jinping's attempt to woo Vietnam on the issue in a state visit.
China, Taiwan, Vietnam, the Philippines, Malaysia and Brunei all have competing claims to various island chains in the South China Sea. Vietnam has lobbied ASEAN countries to take a tougher stand against China as the regional group negotiates a maritime code of conduct.
In August, ASEAN pushed for a legally binding code despite China's insistence for a document with lesser force.
"If I can help mediate or arbitrate, please let me know..." ASEAN meets with China and the US early in the week ahead of a joint meeting with the leaders of nations including the US, China, Japan, Australia, India and Russian Federation.
Unlike China, the U.S. is not a claimant to the potentially oil-rich and busy waters, but it has declared it has a national interest in ensuring freedom of navigation and overflight and the peaceful resolution of the disputes.
Since coming to office 16 months ago, the firebrand leader has been conciliatory to Beijing, despite a ruling by an global arbitration court favoring the Philippines in its territorial dispute with China.
He said the United States and Vietnam boast common benefits to increase cooperation in the time to come.
In a stinging attack on Kim, he reeled off a litany of alleged human-rights abuses and calling him a "deranged tyrant" presiding over a "cult".
Secretary of State Rex Tillerson said Friday he can envision the US and North Korea agreeing to hold talks at some point as a precursor to formal negotiations.