Syria still possesses chemical weapons, US Defence Secretary Jim Mattis has said as he warned against the banned munitions being used again.
Speaking in Tel Aviv, Israel, Mr Mattis said: "There can be no doubt in the international community's mind that Syria has retained chemical weapons in violation of its agreement and its statement that it had removed them all."
Although he did not give details on the amount of weapons held, he added: "I can say authoritatively they have retained some, it's a violation of the United Nations Security Council resolutions and it's going to have to be taken up diplomatically and they would be ill advised to try to use any again, we made that very clear with our strike."
He also said that in recent days the Syrian Air Force has dispersed its combat aircraft - perhaps a result of concerns in Syria about additional US strikes following the cruise missile attack earlier this month in retaliation for alleged Syrian use of sarin gas.
Israeli defence officials said this week that Syria still has up to three tons of chemical weapons in its possession. It was the first specific intelligence assessment of president Bashar Assad's weapons capabilities since a deadly chemical attack earlier this month.
Speaking alongside Mr Mattis, Israeli Defence Minister Avigdor Lieberman also refused to go into detail but said: "We have 100% information that the Assad regime used chemical weapons against rebels".
Assad has strongly denied he was behind the attack in the opposition-held town of Khan Sheikhoun in Syria's northern Idlib province, and has accused the opposition of trying to frame his government.
Russia, a top Assad ally, has asserted a Syrian government air strike hit a rebel chemical weapons factory, causing the disaster.
In response to the April 4 attack, the United States fired 59 missiles at a Syrian air base it said was the launching pad for the attack.
Before meeting with Mr Mattis in Jerusalem, Israeli Prime Minister Benjamin Netanyahu told reporters that Israel is encouraged by the change of administrations in Washington.
"We sense a great change in the direction of American policy," he said.
He referred to the US cruise missile strike in Syria as an important example of the new administration's "forthright deeds" against the use of chemical weapons.
The Syrian government has been locked in a six-year civil war against an array of opposition forces. The fighting has killed an estimated 400,000 people and displaced half of Syria's population.