US Secretary of State John Kerry vowed that the world "will not rest" in the fight against the Islamic State (Isis) as he arrived in Brussels for counter-terrorism talks. Expressing his condolences to the Belgian people following the 22 March attacks he said the Americans were "praying and grieving" with them.

Speaking after a meeting with Belgian Prime Minister Charles Michel, he said the US stands firmly with "Belgium and with the nations of Europe in the face of this tragedy." He said: "United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks."

At least two US citizens were confirmed killed by a senior US official, who told the Associated Press that their families had been informed of their deaths in the attacks at the airport departure terminal and a down town Metro stop that which claimed the lives of 31 people and wounded 270. The official spoke on the condition of anonymity.

Kerry added that the world would not relent in its fight against IS (Daesh). "We – all of us representing countless nationalities – have a message for those who inspired or carried out the attacks here or in Paris, or Ankara, or Tunis, or San Bernardino, or elsewhere: We will not be intimidated," he said. "We will not be deterred. We will come back with greater resolve, with greater strength and we will not rest until we have eliminated your nihilistic beliefs and cowardice from the face of the Earth."

Speaking to reporters after the meeting he said the extremists were resorting to actions outside the Middle East was because their "fantasy of a caliphate is collapsing before their eyes. Its territory is shrinking. Its leaders are decimated. Its revenue sources are dwindling, and its fighters are fleeing."

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