By Roberta Rampton and Manuel Mogato

HANGZHOU, China/VIENTIANE (Reuters) - U.S. President Barack Obama said on Monday he was considering whether or not to meet his Filipino counterpart, Rodrigo Duterte, as planned, amid alarm over the latter's campaign against illegal drugs in which thousands of people have died.

Earlier in the day, the plain-talking Duterte had told reporters he would swear at Obama if he tried to question him about extrajudicial killings, using a Filipino phrase for "son of a bitch" to describe the U.S. president.

Speaking at a G20 meeting in China, and before traveling to Laos for a summit where he was due to hold talks with Duterte, Obama said he had asked his team to find out if "constructive, productive conversations" were possible.

"Obviously the Filipino people are some of our closest friends and allies and the Philippines is a treaty ally of ours. But I always want to make sure that if I'm having a meeting that it's actually productive and we're getting something done," he told reporters in the eastern Chinese city of Hangzhou.

"I'm going to make an assessment ... What is certainly true is that the issues of how we approach fighting crime and drug trafficking is a serious one for all of us, and we've got to do it the right way."

Earlier, Duterte made it clear he would not be lectured on human rights by Obama, adding that "plenty will be killed" before the end of a crackdown that has led to the deaths of about 2,400 people since he became president two months ago.

"I am a president of a sovereign state and we have long ceased to be a colony," Duterte told reporters before setting off for Laos, when asked about his planned meeting with Obama.

"Who is he to confront me? As a matter of fact, America has one too many to answer for," he said. "Everybody has a terrible record of extrajudicial killings."

Asked after arriving in the Laos capital of Vientiane whether the meeting would go ahead, Duterte replied: "Maybe, if I feel good. I don't want to quarrel with him. He's the most powerful president ... on the planet."

Duterte added that he did not believe the subject of extrajudicial killings should be raised at such a summit.

"It is rude."

The former crime-busting mayor of southern Davao city won the presidency in May promising to suppress crime and wipe out drugs and drug dealers.

While his campaign has won popular support, the killings have alarmed rights groups and brought expressions of concern from the United States, a former colonial power and a close Philippine ally, and the United Nations.

ASEAN SUMMIT

The frank public exchanges threaten to overshadow the Association of Southeast Asian Nations (ASEAN) and East Asia Summits in Laos from Tuesday to Thursday.

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

The Philippines has been a key U.S. ally in its dispute with China over the South China Sea, where Washington blames Beijing for militarising a vital global trade route and jeopardizing freedom of movement at sea and in the air.

China rejects those accusations, and in turn blames the United States for 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 invalidated China's vast territorial claims to the waterway after a case was brought by the Philippines, though Beijing refuses to recognize that July ruling.

Since then, the Philippines has sent mixed signals to China. 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.

On Monday, he showed no signs of backing down in his anti-drug campaign.

"Plenty will be killed until the last pusher is out of the streets. Until the (last) drug manufacturer is killed we will continue," he said.

Police say about 900 of those killed died in police operations, and the rest were "deaths under investigation", a term human rights activists say is a euphemism for vigilante and extrajudicial killings.

Duterte earlier 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 in Laos.

In response to a bombing last week that killed 14 people in Duterte's hometown, his government on Monday signed a declaration of a "state of lawlessness", a term that has left many Filipinos concerned that it might be draconian and open to abuse by security forces.

The government, however, has been keen to allay fears the obscure order might impinge on civil liberties. Officials have repeatedly said the declaration simply allows the armed forces to support police operations, such as at checkpoints.

(Additional reporting by Neil Jerome Morales in DAVAO CITY and Michael Martina in HANGZHOU; Editing by Mike Collett-White)