Syrian state TV said on Monday the United States was suspected of striking an air base hours after US President Donald Trump warned of a 'big price to pay' as aid groups said dozens of people were killed by poison gas in a rebel-held town.

The United States denied attacking the Syrian base.

'At this time, the Department of Defense is not conducting air strikes in Syria,' the Pentagon 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.

Syrian state TV said there were casualties in what it said was a suspected US missile attack on the T-4 airfield near Homs, which is close to the ancient city of Palmyra in central Syria.

'An aggression was perpetrated on T-4 air base in several strikes that is most likely to be an American attack,' state television said in a news flash.

A Syrian military source was quoted as saying air defences shot down eight missiles fired at the base.

Defence analysts say there are large deployments of Russian forces at the base, and jets fly regular sorties from there to strike rebel-held areas.

The Syrian state broadcaster said there were several dead and wounded in the strike.

The Syrian opposition blamed the suspected chemical attack on Saturday in the town of Douma on government forces.

As international officials worked to try to confirm the chemical attack, Trump took the rare step of directly criticising Russian President Vladimir Putin in connection with the incident.

Trump said on Sunday there would be a 'big price to pay' after medical aid groups reported dozens of civilians, including many children and women, were killed by poison gas in the besieged rebel-held town of Douma.

'Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay,' Trump wrote on Twitter.

The Syrian government denied its forces had launched any chemical assault. Russia, Assad's most powerful ally, called the reports fake.

The Russian Foreign Ministry warned against military action on the basis of 'invented and fabricated excuses'.

The Syrian government launched a fierce air and ground assault on Douma, the last rebel-held town in the eastern Ghouta district, last Friday.

French President Emmanuel Macron spoke to Trump by telephone and the two agreed that they would work together to establish clear responsibility for what Macron's office said they had agreed was a confirmed chemical attack.

Macron said in February 'France will strike' in the event of lethal chemical weapon attack on civilians by government forces in Syria.

The medical relief organisation Syrian American Medical Society (SAMS) and the civil defence service, which operates in rebel-held areas, said in a joint statement 49 people had been killed in the suspected gas attack.

One video shared by activists showed bodies of about a dozen children, women and men, some with foam at the mouth. 'Douma city, April 7 ... there is a strong smell here,' a voice can be heard saying.

Reuters could not independently verify the reports.

The United States launched a cruise missile strike on a Syrian air base a year ago in response to the killing of dozens of civilians in a sarin gas attack in an opposition-held town in northwest Syria. The gas attack was blamed on Assad.

US government sources said Washington's assessment of the Saturday attack was that chemical weapons were used.

The European Union also said evidence pointed to the use of chemical weapons by Assad's forces.

A European diplomat said Western allies would work on building a dossier based on photos, videos, witness testimony and satellite images of Syrian flights and helicopters. However gaining access to samples on the ground would be difficult.

The UN Security Council will meet twice on Monday following rival requests by Russia and the United States.

UN war crimes investigators had previously documented 33 chemical attacks in Syria, attributing 27 to the Assad government, which has repeatedly denied using the weapons.