US President Donald Trump will skip an upcoming summit in South America and will remain in the US to “oversee the American response to Syria and to monitor developments around the world”.

Russia vetoed a United Nations bid to set up a Syria chemical weapons enquiry just hours after White House spokeswoman Sarah Huckabee Sanders said Mr Trump would not attend the Summit of the Americas in Lima, Peru or travel to Bogota, Colombia as planned.

Vice President Mike Pence will travel in his place. It’s scheduled to begin on April 13.

Trump said on Monday he would “forcefully” respond to an alleged chemical weapons attack in Syria and would soon decide how.

The White House made the announcement about the schedule change a day after federal agents raided the office of Mr Trump’s personal lawyer.

It also came as Homeland Security Adviser Tom Bossert, who had been part of the Trump administration since the president took office, was forced to resign, reportedly by new national security adviser John Bolton.

“The President is grateful for Tom’s commitment to the safety and security of our great country. Tom led the White House’s efforts to protect the homeland from terrorist threats, strengthen our cyber defences, and respond to an unprecedented series of natural disasters. President Trump thanks him for his patriotic service and wishes him well,” White House Press Secretary Sarah Sanders said in a statement.

Bloomberg reported that Mr Bolton forced Mr Bossert out of his role, having taken up his role as National Security Adviser just yesterday.

The departure was a huge shock as Mr Bossert had just been out doing the Sunday morning political TV shows talking about a range of topics, including Syria and Mr Trump’s decision to send the National Guard to the Mexico border.

On Syria, Mr Bossert questioned the “timing” of the alleged chemical weapons attack in Syria, but he called it “a quite serious problem” and said “all options” remain on the table.

He said the attack falling exactly one year after the US bombed a Syrian air base over another suspected use of chemical weapons in rebel-held territory “struck” him, but the era where the US acted as world police is over.

“I think the president’s got a point that’s been very clear,” he said. “For too long and the United States of America has been taken advantage of in their responsibility to provide security for the entire world ... putting their resources and their treasure and their boys and girls on the line ... American troops aren’t going to fix the six or seven different ongoing conflicts and wars going on in the Middle East or in Syria at this stage.”

On the National Guard, Mr Bossert defended Mr Trump’s decision to send troops to the Mexico border.

“We’ve got a leaking boat on our border. And we’re all quibbling with how much water’s in the boat and how fast we’re bailing it out,” Mr Bossert told US ABC’s program This Week on Sunday. “I think at this point the president’s been pretty clear, enough is enough, fix the actual problem and stop that leak.”

RUSSIA VETOES SYRIA CHEMICAL WEAPONS INQUIRY

Meanwhile, Russia has vetoed a US resolution to create a new expert body to determine responsibility for chemical weapons attacks in Syria.

The UN Security Council held a vote at 5AM AEST on a US draft resolution to establish a new inquiry into who is to blame for using chemical weapons in Syria, following the alleged toxic gas attack in Douma last week.

It was the 12th time that Russia has used its veto power at the council to block action targeting its Syrian ally.

The United States circulated a revised draft resolution to the 15-member council, which it first put forward on March 1, amid a warning from Mr Trump that there would be a “big price to pay” for a suspected deadly poison gas attack on a rebel-held Syrian town on Saturday.

The resolution needed nine votes in favour and no vetoes by Russia, China, France, Britain or the United States to pass.

The Syrian government has invited the Organisation for the Prohibition of Chemical Weapons to send a team to investigate allegations of a chemical attack in the eastern Ghouta town of Douma.

The suspected chemical weapons attack late on Saturday killed at least 60 people, with more than 1000 injured at several sites in Douma, a town near the capital, Damascus, according to the Union of Medical Care Organisations.

The Syrian government and Russia have denied involvement in any such attack.

“Syria is keen on co-operating with the OPCW to uncover the truth behind the allegations that some western sides have been advertising to justify their aggressive intentions,” state news agency SANA said, quoting an official source in the foreign ministry.