VIENTIANE (Reuters) - (Note: paragraphs 1, 6 and 19 contain language that may offend some readers)

New Philippines President Rodrigo Duterte sought to defuse a row with the United States on Tuesday, voicing regret for calling President Barack Obama a “son of a bitch”, a comment that prompted Washington to call off a bilateral meeting.

The tiff between the two allies overshadowed the opening of a summit of East and Southeast Asian nations in Vientiane, Laos.

It also soured Obama’s last swing as president through a region he has tried to make a focus of U.S. foreign policy, a strategy widely seen as a response to China’s economic and military muscle-flexing.

Diplomats say strains with longtime ally the Philippines could compound Washington’s difficulties in forging a united front with Southeast Asian partners on the geostrategic jostle with Beijing over the South China Sea.

Duterte has bristled repeatedly at criticism over his “war on drugs”, which has killed about 2,400 people since he took office two months ago, and on Monday said it would be “rude” for Obama to raise the question of human rights when they met.

Such a conversation, Duterte told reporters, would prompt him to curse at Obama, using a Filipino phrase “putang ina” which can mean “son of a bitch” or “son of a whore”.

He has previously used the epithet against Pope Francis, although he later apologized, and the U.S. ambassador to the Philippines.

After Washington called off Tuesday’s bilateral meeting between Obama and Duterte in response to his latest comment, the Philippines issued two statements expressing regret and also briefed reporters.

“President Duterte explained that the press reports that President Obama would ‘lecture’ him on extrajudicial killings led to his strong comments, which in turn elicited concern,” the Philippines government said in a statement.

“He regrets that his remarks to the press have caused much controversy,” it added. “He expressed his deep regard and affinity for President Obama and for the enduring partnership between our nations.”

Obama’s deputy national security adviser, Ben Rhodes, said the focus on Duterte’s comments leading into the summit had not created a constructive environment for a bilateral meeting.

“All of the attention frankly was on those comments, and therefore not on the very substantive agenda that we have with the Philippines,” he told reporters.

Officials from both countries said there would be no formal meeting rescheduled in Laos but a short conversation between the two presidents was possible.

Instead of the Duterte meeting, Obama held talks with South Korean President Park Geun-hye, a day after North Korea fired three medium-range missiles into the sea. He urged a full implementation of sanctions against North Korea, adding that the missile test demonstrated the threat that Pyongyang posed.

Obama is also likely to hold an unscheduled meeting in Laos with Japanese Prime Minister Shinzo Abe to discuss North Korea, Rhodes said. He said Washington needed to maintain a sense of urgency within the international community on sanctions against Pyongyang.

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MOVES TO SOOTHE TENSIONS

Obama, the first sitting U.S. president to visit Laos, said on Tuesday he wanted to address the legacy of U.S. bombing during the Vietnam War. He announced that Washington would provide an additional $90 million over three years to help clear unexploded ordnance, which has killed or wounded over 20,000 people.

But the unusually open tensions between the United States and the Philippines, its former colony, threaten to overshadow the Association of Southeast Asian Nations (ASEAN) and East Asia Summits in Laos, which run until Thursday.

The 10-member ASEAN will also meet leaders of other regional powers: China, Japan, South Korea, Australia, India, Russia and the United States.

Duterte won the presidency in May after promising to suppress crime and wipe out the illegal drug trade in a country where the number of methamphetamine users is estimated to be at least 1.3 million in a population of 100 million.

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About 900 people linked to drugs have been killed in police operations since July 1 and a further 1,500 have been classified as “deaths under investigation”, a term human rights activists in the Philippines say is a euphemism for extrajudicial killings.

Duterte has poured scorn on critics of his uncompromising campaign, usually larding it with curses.

He lambasted the United Nations after it criticized the surge in killings and he turned down a meeting with U.N. Secretary General Ban Ki-moon at the Laos summit.

He has accused a senator heading an inquiry into the killings of getting payoffs from drug lords.

Manila has been aligned with the United States in its dispute with China over the South China Sea, in which Washington blames Beijing for militarizing a vital global trade route and jeopardizing freedom of movement at sea and in the air.

China rejects those accusations and accuses the United States of ratcheting up tensions unnecessarily. China claims most of the South China Sea, through which more than $5 trillion of trade moves annually. Brunei, Malaysia, the Philippines, Taiwan and Vietnam have rival claims.

An arbitration court in The Hague in July invalidated China’s territorial claims after a case was brought by the Philippines, a ruling Beijing refuses to recognize.

Duterte said last month he expected all ASEAN members to support the arbitration court’s ruling, but that the Philippines would not raise the issue in Laos.