UN chief says US' Syria warning 'should be heard'

A U.S. warning to Syrian President Bashar al-Assad "should be heard", the head of the United Nations said Wednesday.

"I believe it was a serious warning, and I think serious warnings should be heard," Antonio Guterres told reporters before meeting Secretary of State Rex Tillerson at the State Department.

The U.S. on Monday said it had "identified potential preparations" for a new chemical weapons attack, warning that should Assad's forces launch an attack "he and his military will pay a heavy price".

White House spokesman Sean Spicer said the activities are "similar" to preparations the Syrian regime made before a chemical weapons attack in April by warplanes in Idlib that killed more than 100 civilians.

On Tuesday, the U.S.'s UN ambassador widened the scope of the warning to include Assad's principal international backers -- Russia and Iran.

"If this happens again, we are putting you on notice. And my hope is that the president's warning will certainly get Iran and Russia to take a second look, and I hope that it will caution Assad from the fact that we don't want to see innocent men, women and children hurt again," Nikki Haley said.

Moscow dismissed the U.S. claims Wednesday, calling suggestions that Syrian forces are preparing for another attack a "provocation".

Russia also warned Washington against further "irresponsible" actions in violation of the UN Charter and generally recognized norms of international law, citing the U.S. missile strike on a Syrian air base in response to the Idlib attack.

The U.S. has yet to make public any evidence to substantiate its claim.