Trump's national security team meets as US says it has proof Syria carried out chemical attack President Trump has been in discussions with allies, the White House says.

President Donald Trump's national security team met Friday evening at the White House as Washington and the world waited for word of any decision on a military response to Syria's alleged chemical attack last weekend.

The president has been promising a decision since Monday and on Wednesday he tweeted that Russia should "get ready" because missiles "will be coming."

The White House meeting Friday came as UN ambassador Nikki Haley asserted the U.S. has "proof" that the Syrian regime had carried out the chemical attack.

As of Friday evening, the White House had not provided any readout of the "deputies level" national security meeting, but White House deputy press secretary Raj Shah told CNN that the president has also been in constant communication with allies, including the British and French, expected to partner with U.S. military forces in the event of a strike.

Haley said that the president would not be rushed into a decision.

"If you rush decisions like this you make a mistake," Haley said.

Haley also said was no uncertainty about whether there was, in fact, a chemical attack, which the Russians, allied with Syrian President Bashar al Assad, have denied.

“Did a chemical weapons attack happen? Yes,” Haley said, speaking to reporters just before a meeting of the Security Council earlier in the day in New York. “The U.S. has analyzed, yes it has happened. The UK has analyzed, yes, it has happened. And France has analyzed, yes, it has happened. Three countries.”