Deadly missile strikes hit a Syrian military airbase state media said, but Washington and Paris denied carrying them out in response to an alleged poison gas attack.
US President Donald Trump and French counterpart Emmanuel Macron had on Sunday vowed a "strong, joint response" to the suspected chemical attack on the rebel-held town of Douma.
The alleged gas attack sparked global alarm and the UN Security Council was expected to discuss the crisis later on Monday.
Syrian state news agency SANA reported just before dawn that "several missiles" had hit T-4, also known as the Tiyas military base, in the central province of Homs.
It said air defence systems had been activated and initially reported it as a "suspected US attack" but later withdrew all references to the United States.
SANA said there were dead and wounded but did not give specific figures.
The Syrian Observatory for Human Rights, which monitors the country's conflict, said 14 fighters had been killed, including Iranian forces allied to the country's regime.
Forces from regime allies Russia and Iran, as well as fighters from the Lebanese Hezbollah militia, are known to have a presence at the base, said Observatory chief Rami Abdel Rahman.
In Washington, a Pentagon spokesman denied Washington was behind the strikes.
"At this time, the Department of Defence is not conducting air strikes in Syria," the spokesman said.
"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."
US forces a year ago fired a volley of cruise missiles at the government's Shayrat air base in retaliation for another suspected chemical attack in April 2017.
The Shayrat airport lies just 70 kilometres (45 miles) west of T-4 along a main highway.
French armed forces spokesman Colonel Patrik Steiger also denied France carried out the strike, telling AFP: "It was not us."
A military spokeswoman for Israel, which has struck Syrian military positions several times in recent years, declined to comment on the strike.
Israel has repeatedly warned that it will not accept its arch-foe Iran entrenching itself militarily in Syria. In February it carried out major air raids in Syria after an Israeli F16 fighter was shot down by Syrian air defences.
AFP's correspondent in eastern Lebanon said a plane could be heard flying overhead towards the eastern border with Syria around 3:30 am local time (0030 GMT).
The T-4 base lies between Syria's third city Homs and the ancient town of Palmyra.
Trump had reacted with fury to Saturday's alleged chemical attack in Douma -- the last rebel-held area of the onetime opposition enclave of Eastern Ghouta -- lashing out at Assad and Russian President Vladimir Putin.
"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay," Trump warned on Sunday.
Syria and Moscow denounced the allegations as "fabrications" and warned against them being used as a pretext for any military action.
Backed by Moscow, Assad has waged a seven-week assault to dislodged rebels from Eastern Ghouta, their former bastion on the edge of Syria's capital.
The assault killed more than 1,700 civilians, displaced tens of thousands, and left Islamist rebels cornered in their last holdout of Douma, the main town in Ghouta.
Late Saturday, the White Helmets, which act as first responders in opposition-held areas of Syria, said "poisonous chlorine gas" had affected hundreds in Douma.
In a joint statement with the Syrian American Medical Society, the rescue force said at least 48 people had died, bearing symptoms consistent with exposure to a toxic substance.
Footage posted by the White Helmets, which was not possible to verify, showed victims with yellowed skin crumpled on the ground and foaming at the mouth.
Relief workers inside Douma told AFP dead bodies still lay in the streets on Sunday and hospitals were teeming with dead and wounded.
After capturing most of Ghouta with a military assault, Syria and its ally Russia secured two negotiated withdrawals last month that saw 46,000 rebels and civilians leave the enclave.
A preliminary deal for the third and final pocket of Douma saw hundreds of civilians and rebels from Jaish al-Islam leave the town last week.
After days of talks and a respite from bombing, negotiations collapsed and strikes resumed Friday, killing nearly 100 people, according to the Observatory.
On Sunday, state media announced a deal was agreed for Jaish al-Islam to leave Douma within 48 hours and release hostages it was holding.
Several buses of detainees arrived in Damascus overnight after having been freed by Jaish al-Islam, said state news agency SANA.
It said early on Monday that other buses carrying rebels and civilians had begun leaving Douma as well.
Moscow said some 8,000 fighters and 40,000 civilians would be bussed out and said that proved "no chemical weapons were used in this area".
Jaish al-Islam has not confirmed the deal, but a civilian committee from Douma participating in the talks said a "final agreement" was reached for rebels to leave.