BRUSSELS/ISTANBUL/BEIRUT: U.S. Defense Secretary Jim Mattis said Wednesday that the Syrian regime of President Bashar Assad appeared so far to have heeded a warning this week from Washington not to carry out a chemical weapons attack.

Russia, the Syrian government’s main backer in the country’s civil war, warned that it would respond with dignity and proportionately if the United States took pre-emptive measures against Syrian forces to stop what the White House says could be a planned chemical attack.

The White House said Monday that it appeared the Syrian military was preparing to conduct a chemical weapons attack and said that Assad and his forces would “pay a heavy price” if it did so.

The warning was based on intelligence that indicated preparations for such a strike were underway at the Shayrat airfield, U.S. officials said.

“It appears that they took the warning seriously,” Mattis said. “They didn’t do it,” he told reporters flying with him to Brussels for a meeting of NATO defense ministers. He offered no evidence other than the fact that an attack had not taken place.

Asked whether he believed Assad’s forces had called off any such strike completely, Mattis said: “I think you better ask Assad about that.”

Washington accused Syrian forces of using the Shayrat airfield in Homs province for a chemical weapons attack in April. Syria denies this.

The intelligence that prompted the administration’s warning to Syria this week was “far from conclusive,” said a U.S. official familiar with it. “It did not come close to saying that a chemical weapons attack was coming,” the official said.

The intelligence consisted of a Syrian warplane being observed moving into a hangar at the Shayrat air base, where U.S. and allied intelligence agencies suspect the Assad government is hiding chemical weapons, a second U.S. official said.

The Syrian military and Foreign Ministry did not comment on the White House warning, although state-run Al-Ikhbariya television station said the claims were fabricated.

Russian Foreign Minister Sergey Lavrov said Wednesday that Moscow will respond if the United States takes measures against Syrian government forces.

Speaking at a news conference with his German counterpart, Lavrov said he hoped that the United States was not preparing to use its intelligence assessments about the Syrian government’s intentions as a pretext to mount a “provocation” in Syria.

On the ground, meanwhile, at least 30 civilians were killed in airstrikes on the Daesh-held village of Dablan in Deir al-Zor province, the Britain-based Syrian Observatory for Human Rights said.

The Observatory said it was not immediately able to say whether the strikes were carried out by the U.S.-led coalition, or by the Syrian army or its Russian ally. In other developments, Israeli forces struck a Syrian army target in eastern Al-Samadaniya in Qunaitra countryside, the Hezbollah-run military media unit run said. No casualties were reported. The Israeli military said the Israeli strike was in retaliation to errant fire from the Syrian side that hit the occupied Golan Heights, according to newspaper Haaretz.

Meanwhile, Turkey said its artillery bombarded and destroyed Kurdish YPG militia targets after the group’s fighters opened fire on Turkish-backed forces in northern Syria.

The Turkish army said YPG machine-gun fire Tuesday evening targeted Turkish-backed Free Syrian Army rebels south of the town of Azaz. Artillery struck back in retaliation, a Turkish military statement said.

In Geneva, the U.N. human rights chief said at least 173 civilians have been killed in operations against Daesh in Raqqa this month, while as many as 100,000 civilians are trapped in the city.