Defense Secretary Jim Mattis said "nothing is certain" when asked whether he believed the strikes would deter Syrian President Bashar Assad from using chemical weapons again.
Inspectors from the Organisation for the Prohibition of Chemical Weapons have met Syrian Deputy Foreign Minister Faisal Mekdad.
The United States, Britain and France fired more than 100 missiles at Syria early on Saturday in the first coordinated Western military intervention against the Damascus government. Russian Federation backs Syrian President Bashar al-Assad's government.
The inspectors will face a hard task, with all key players having pre-empted their findings, including Western powers, which justified the strikes by claiming they already had proof such weapons were used. The strikes were in retaliation for a suspected chemical weapons attack in Douma, near Damascus.
Douma was the final rebel holdout in an nearly two-month regime assault to reconquer the last opposition bastion outside Damascus.
Jasmine el-Gamal, the Pentagon's Syria country director during the Obama administration, told NPR's Weekend Edition on Sunday that the strikes were successful because they showed that President Trump was following through on his commitment.
That drew derision from his critics and parallels with president George W. Bush's notoriously premature Iraq war victory speech on an aircraft carrier nearly exactly 15 years ago.
The two leaders "found that this illegal action seriously damaged the prospects of a political settlement in Syria", a Kremlin statement said.
Damascus and its ally Russian Federation have denounced the Western action, in particular for refusing to wait until the results of the fact-finding mission sent by the OPCW in the wake of the April 7 incident.
More than 350,000 people have been killed in Syria's war since it started in 2011 with the brutal repression of anti-government protests.
Russia, which is helping Assad fight militants and rebels opposed to his rule, immediately condemned the strikes and called for an emergency meeting of the UN Security Council.
Ambassadors to the United Nations during a Security Council vote on the suspected Syrian chemical attack.
On April 1, gunfire wounded six people on a convoy of buses ferrying opposition fighters and civilians from Eastern Ghouta towards the northwestern province of Idlib, the Observatory said.
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