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President Donald Trump offered his services Sunday to help resolve territorial disputes in the South China Sea.

Speaking in Vietnam, Trump said he was “a very good mediator and arbitrator” and could help to make progress on the regional dispute, in which five countries contest China’s sweeping claims to the waters.

Vietnam has become the most vocal critic of China’s claims to the sea, as well as its construction and militarization of artificial islands in the busy waterway. Some $3-trillion in goods passes through the sea each year, according to Reuters.

"If I can help mediate or arbitrate, please let me know," Trump said in comments before a meeting in Hanoi with Vietnam's president, Tran Dai Quang. He acknowledged that China's position on the South China Sea was a problem.

President Quang said Vietnam believed in handling disputes on the South China Sea through peaceful negotiations and on the basis of international laws — which Vietnam says nullify China's claims, the news agency reported.

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Related: Trump Mocks North Korean Dictator Kim Jong Un Over ‘Old’ Remark

Vietnam has reclaimed land around reefs and islets, but on nowhere near the same scale as China. Brunei, Malaysia, the Philippines and Taiwan also have claims in the sea.

Since Philippine President Rodrigo Duterte has grown closer to China, Vietnam has emerged as China's main challenger in the region. In July, China pressured Vietnam to stop oil drilling in a disputed area, taking relations to a new low.

Relations have since improved and Chinese President Xi Jinping was visiting Hanoi later on Sunday.

President Donald J. Trump and Vietnamese President Tran Dai Quang at the Presidential Palace in Hanoi, Vietnam, on Sunday. KHAM / Pool / EPA

The South China Sea was discussed in Beijing on an earlier leg of Trump's 12-day Asian tour and U.S. Secretary of State Rex Tillerson said the United States and China had a frank exchange of views.

The United States has angered China with freedom of navigation patrols close to Chinese-controlled islands.

Trump's remarks came shortly after he tweeted a series of insults about North Korean leader Kim Jong Un and critics of his relationship with Russia.

On Saturday North Korea slammed Trump accusing him of being a “destroyer” and having “begged” for nuclear war during his first trip to Asia.

Trump later responded suggesting Kim Jong Un was “short and fat,” adding “oh well, I try so hard to be his friend — and maybe someday that will happen.”

Trump also lashed out at all the “haters and fools” who he said need to realize that “having a good relationship with Russia is a good thing.”

“There [sic] always playing — politics — bad for our country. I want to solve North Korea, Syria, Ukraine, terrorism, and Russia can greatly help!”