WASHINGTON (REUTERS) - Islamic State in Iraq and Syria (ISIS) has been defeated in Syria, United States Vice-President Mike Pence said on Wednesday (Jan 16), hours after American soldiers were killed in a northern Syria bomb attack claimed by the militant group.

Mr Pence did not mention the deaths in an address to 184 chiefs of US diplomatic missions gathered in Washington from around the world for a speech that sounded more like a campaign rally than a strategic overview of US foreign policy.

"The caliphate has crumbled and ISIS has been defeated," Mr Pence told the US ambassadors and other senior American diplomats, referring to ISIS.

Earlier, a spokesman for Mr Pence's office, Ms Alyssa Farah, said the vice-president had been briefed about the soldiers' deaths and expressed his sympathy.

There were mixed reports about how many Americans died in the blast in the northern Syria town of Manbij, with one US official saying four were killed while others said two had died.

Turkish President Tayyip Erdogan said 20 people were killed, including five US troops.

A US official who declined to be named said four US troops had been killed and three wounded in the blast, which an ISIS-affiliated site said was the work of a suicide bomber.

A war monitor said 19 people in total had died in the blast.

The attack comes nearly a month after US President Donald Trump's surprise Dec 19 announcement that he would withdraw 2,000 US troops from Syria after concluding that ISIS had been defeated there.

His decision led to the resignation of US Defence Secretary Jim Mattis, who cited policy differences with the president for his leaving.

LACK OF PROGRESS

Some US diplomats, speaking on condition of anonymity, said they were surprised and dismayed by the speech by Mr Pence, who often paused during his remarks as if to invite applause that appeared to grow fainter as he spoke.

"We're not used to being at campaign rallies," said one US official.

Despite talks of a second leaders' summit between Mr Trump and North Korea's leader Kim Jong Un, Mr Pence acknowledged that efforts to convince Pyonyang to give up its nuclear arsenal had not made headway.

"While the president is promising dialogue with Chairman Kim, we still await concrete steps by North Korea to dismantle the nuclear weapons that threaten our people and our allies in the region," he said.

The vice-president also criticised China's "unfair" trade practices and loans to developing countries that pushed up their debt levels as it tries to gain greater influence in the world.

"The truth is that too often in recent years, China has chosen a path that disregards the laws and norms that have kept the world state prosperous for more than half a century," he said.

"The days of the United States looking the other way are over," he added.

Mr Pence said the administration's foreign policy was based on Mr Trump's "America First" agenda.

"No longer will the United States government pursue grandiose, unrealistic notions at the expense of American people," he said.

He acknowledged that Mr Trump's foreign policy was "different from what the world has come to expect" and that the US faced different threats than during the Cold War.

"Today, we are not up against one super power but several great powers competing with us for pre-eminence across the world," he said, saying the US faced a "wolf pack" of rogue states including Iran, Cuba, Venezuela and Nicaragua.