The Pentagon said it destroyed three chemical weapons facilities of the Syrian regime.
Marine Lt Gen Kenneth F McKenzie director, Joint Staff, told reporters that the US, France and Britain in all deployed 105 missiles against three targets.
"Taken together were able to overwhelm the Syrian air defence system," he said.
The US assesses that the Syrians fired about 40 missiles in retaliation, but none of them endangered the US forces and its allies. Defensive effort of Syria was ineffective, he said.
Pentagon Chief Spokesperson Dana White alleged that the Syrian regime had a pattern of using chemical weapons against the chemical weapons convention.
"This is a heinous regime, which murders its people daily," she said.
It was a deliberate decision to the target the storage and research and development facilities, she said.
"We are confident that we have significantly degraded the ability to ever use the chemical weapons again," White said, asserting that the US policy on Syria has not changed.
"Our mission in Syria remains the same, to defeat the Islamic State and not be involved in the civil war," White said, adding that the US is 100 per cent behind the Geneva peace process.
The new UN envoy provides an opportunity for everyone to work towards a Syrian peace process.
"We are confident that the UN process will move forward," White told reporters in response to a question.
"We took action. What happens next is in the hands of Assad," she said.
The strikes went to the "very heart" of the chemical weapons programme of the Assad regime, the Pentagon said, exuding confidence that they "significantly degraded" their ability to ever use chemical weapons again.
"We were very methodical in making the decision about these sites. And it was a deliberate decision to go to the storage facilities, to go to the research and development facilities.
"We think by doing this, this was very successful, and we are confident that we've significantly degraded his ability to ever use chemical weapons again," White said.
"We did this because it's intolerable for any civilised nation to tolerate the use of chemical weapons. We believe by hitting Barzeh first, we've attacked the heart of the Syrian chemical weapons programme.
"I'm not saying that they're going not be able to reconstitute it, not saying that it's going to continue. This has dealt them a very serious blow, so I think that's the core of what I'm saying," she said.
The US did no coordination with the Russians, McKenzie said.
Responding to questions, he said it is possible that some people might have left from the strike facilities. The strikes were carried out in the week hours of the morning (3am-4am) so as to reduce the possibility of any civilian casualty.
The US believes that the material and equipment at the source of this site was not movable.
"This is far more damaging to Syria," he said.
"It's also important to note that we flew a variety of defensive counterair, tanker and electronic warfare aircraft in support of these operations. None of our aircraft or missiles involved in this operation were successfully engaged by Syrian air defenses, and we have no indication that Russian air-defense systems were employed," he said.
Confident that all of their missiles reached their targets, he said at the end of the strike mission, all aircraft safely returned to their bases.
The Pentagon also assessed that defensive efforts of Syria were largely ineffective, and clearly increased risk to their people based on this indiscriminate response.
"When you shoot iron into the air without guidance, it's going to come down somewhere. By contrast, the precise nature of our strike and the care which our allied team planned and executed significantly reduced the risk of collateral damage to civilians," he said.
White said this was a fully legitimate operation and the President had the authority to order conducting these strikes.
"It was a successful mission. What happens next depends on what the Assad regime decides to do," she added.
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