The Pentagon has denied US forces are behind a missile strike on an airfield near the Syrian city of Homs.
Syrian state television reported that loud explosions were heard near the T-4 airfield, and a Syrian military source was reported as saying eight missiles had been shot down.
Defence analysts said the base hosts large deployments of Russian forces and combat aircraft which fly regular sorties to strike rebel-held areas.
Syrian state TV said there were several dead and wounded.
“An aggression was perpetrated on T-4 airbase in several strikes that is most likely to be an American attack,” state television said in a news flash.
Earlier US President Donald Trump warned there would be a “big price to pay” after medical aid groups reported dozens of people were killed by poison gas in a besieged rebel-held town in Syria.
But the Pentagon said the US was not behind the airfield attack.
“At this time, the Department of Defence is not conducting air strikes in Syria,” it said in a statement.
“However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable.”
When asked about the explosions, an Israeli spokeswoman declined to comment.
Israel has struck Syrian army locations many times in the course of the conflict, hitting convoys and bases of Iranian-backed militias that fight alongside Syrian President Bashar al-Assad’s forces.
In February, Israeli warplanes targeted areas near the T-4 airbase during a bombing raid on Iran-backed positions.
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Syria denied it had launched any such attack, a denial which was backed by its ally Russia.
The Russian Foreign Ministry warned against any retaliatory military action on the basis of “invented and fabricated excuses”, saying it could lead to severe consequences.
In April 2017, Mr Trump ordered a cruise missile strike on Syria’s Shayrat air base in retaliation for a chemical weapons attack against civilians in the town of Khan Sheikhoun.