US President Donald Trump and his team discussed three options before zeroing in on a limited missile strike in Syria in response to a "heinous" chemical attack on civilians by President Bashar Assad's regime, his aides said today.


"There were three options we discussed with the president, and the president asked us to focus on two options in particular, to mature those options, and he had a series of questions for us that we endeavoured to answer," US National Security Advisor Lt Gen H R McMaster told reporters during a hurriedly convened news conference.


"After a meeting of considerable length and a far-reaching discussion, Trump decided to act," McMaster said giving the sequence of events.


He said the national security team weighed the risk associated with any military action. "But we weighed that against the risk of inaction, which is the risk of this continued, egregious, inhumane attacks on innocent civilians with chemical weapons," McMaster said at the news conference which was also addressed by Secretary of State Rex Tillerson.


"It's important to recognise, as I think everyone does, the chaotic circumstances that exist on the ground in Syria with the presence of a battle underway to defeat ISIS, the presence of Al-Qaeda elements inside of Syria and a civil war that is underway," he said.


"We feel that the strike itself was proportional because it was targeted at the facility that delivered this most recent chemical weapons attack," he said.


According to McMaster, Trump was immediately notified after the chemical attack that killed 86 people, including 27 children. "He was very interested in understanding better the circumstances of the attack and who was responsible," he noted.