After threatening a military strike against Syria, President Donald Trump on Tuesday cancelled plans to travel to South America later this week, choosing to stay in the United States to manage the response to an apparent chemical weapons attack.

White House spokeswoman Sarah Huckabee Sanders said Tuesday that Trump will not attend the 8th Summit of the Americas in Lima, Peru or travel to Bogota, Colombia as planned, remaining in the United States to "oversee the American response to Syria and to monitor developments around the world."

The decision marks the first time an American president has not attended the summit. Vice President Mike Pence will travel in Trump's place.

Trump on Monday promised a decision on Syria within hours, declaring that Russia or any other nation found to share responsibility for Saturday's apparent chemical weapons attack on civilians will "pay a price."

The White House sharply rejected any suggestion that Trump's own words about pulling U.S. troops out of Syria had opened the door for the attack, which killed more than 40 people, including children.

Trump, asked whether Russian President Vladimir Putin bore any responsibility, responded, "He may, yeah, he may. And if he does it's going to be very tough, very tough." He added, "Everybody's gonna pay a price. He will. Everybody will."

Amid the tough talk from the White House, the U.S. military appeared to be in position to carry out any attack order. A Navy destroyer, the USS Donald Cook, was underway in the eastern Mediterranean after completing a port call in Cyprus. The guided missile destroyer is armed with Tomahawk cruise missiles, the weapon of choice in a U.S. attack one year ago on an airfield in Syria following an alleged sarin gas attack on civilians.

The Russian military, which has a presence in Syria as a key Assad ally, said its officers had visited the weekend site in a suburb of Damascus, the Syrian capital, and found no evidence to back up reports of poison gas being used. Russia's U.N. ambassador, Vassily Nebenzia, accused Washington of deliberately stoking international tensions by threatening Russia in a tone "beyond the threshold of what is acceptable, even during the Cold War."

Trump said there was little question that Syria was responsible for the apparent weekend attack, although the government of President Bashar Assad denied it. "To me there's not much of a doubt, but the generals will figure it out," Trump said.

He promised a decision on a possible military response within 24 to 48 hours, "probably by the end of today."

Emphatic in his condemnation of the apparent gas attack, Trump noted graphic pictures of the dead and sickened, calling the assault "heinous," ''atrocious," ''horrible" and "barbaric."

Fielding questions at the White House, Trump press secretary Sarah Sanders said it would be "outrageous" to say that Trump's recent announcement that he intends to remove all U.S. forces from Syria in the coming months had emboldened Assad. "I think that it is outrageous to say that the president of the United States green-lit something as atrocious as the actions that have taken place over the last several days," she said.

Defense Secretary Jim Mattis, in separate remarks at the Pentagon, also suggested Moscow bore some blame. He criticized Russia for what he suggested was its failure to ensure the elimination of Syria's chemical weapons arsenal under terms of a 2013 agreement.

Trump was to meet late in the day with senior national security aides, and no action was "off the table," the president said. Monday was the first day on the job for his new national security adviser, John Bolton, who has previously advocated military action against Syria.

Trump said, "If it's Russia, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out."

The United States, meanwhile, urged the U.N. Security Council to adopt a resolution that would condemn the continuing use of chemical weapons in Syria "in the strongest terms" and establish a new body to determine responsibility for chemical attacks. The draft resolution, obtained by The Associated Press, was circulated ahead of an emergency Security Council meeting.

An American official said the U.S. was discussing with allies whether they would participate in a retaliatory strike. If Trump decides to proceed quickly, the most likely partner would be France rather than Britain, because of concerns about obtaining permission from Parliament, said the official, who wasn't authorized to discuss the planning publicly and requested anonymity.

As U.S. officials consider whether and how to respond, they are looking at what type of chemical agent might have been used. When Trump ordered airstrikes last year after a chemical weapons attack, it was a response to the use of sarin gas, which is banned by the Chemical Weapons Convention that Syria has signed. An attack with chlorine, which can be used as a weapon but is not outright banned by the treaty, could raise precedent issues, as there have been numerous recent allegations of chlorine attacks in Syria that have drawn no response from the Trump administration.

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AP writers Catherine Lucey, Josh Lederman, Edie Lederer and Jonathan Lemire contributed.

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