TEL AVIV, Israel -- Syria still possesses chemical weapons, U.S. Defense Secretary Jim Mattis said in Israel on Friday, warning against the banned munitions being used again.

At a news conference in Tel Aviv, Mattis also said that in recent days the Syrian Air Force has dispersed its combat aircraft. The implication is that Syria may be concerned about additional U.S. strikes following the cruise missile attack earlier this month in retaliation for alleged Syrian use of sarin gas.

Mattis spoke alongside Israeli Defense Minister Avigdor Lieberman. “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,” said Mattis. He said he didn’t want to elaborate on the amounts Syria has in order to avoid revealing sources of intelligence.

“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 said.

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Israeli defense 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.

Lieberman also refused to go into detail but said “We have 100 percent information that 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. Top Assad ally, Russia, has asserted a Syrian government airstrike 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 Mattis in Jerusalem, 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,” Netanyahu said. He referred to the U.S. 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.

Israel has largely stayed out of the fighting, though it has carried out a number of airstrikes on suspected Iranian weapons shipments it believed were bound for the Lebanese militant group Hezbollah. Iran and Hezbollah, both bitter enemies of Israel, along with Russia have sent forces to support Assad.

Syria agreed to give up its chemical weapons arsenal to avert U.S. strikes following a chemical weapons attack in opposition-held suburbs of Damascus in August 2013 that killed hundreds of people and sparked worldwide outrage.

Ahead of that disarmament, Assad’s government disclosed it had some 1,300 tons of chemical weapons, including sarin, VX nerve agent and mustard gas.

The entire stockpile was said to have been dismantled and shipped out under international supervision in 2014 and destroyed. But doubts began to emerge soon afterward that not all such armaments or production facilities were declared and destroyed. There also is evidence that the Islamic State group and other insurgents have acquired chemical weapons.