Residents of the Syrian town devastated by a chemical weapons attack last week said warplanes had returned to bomb them, despite a US missile barrage and warnings of possible further response.

At least 86 people in the northwestern town of Khan Sheikhoun were killed in a chemical attack that left hundreds choking, gripped by spasms or foaming at the mouth. Eyewitnesses and a monitoring group, the Syrian Observatory for Human Rights, said yesterday that fresh attacks on the area - now a virtual ghost town - had killed one woman.

Residents cowered in bedrooms and basements throughout yesterday, underscoring the apparently unchanged threat they faced from the Syrian Government's arsenal of rockets, barrel bombs and other weapons that have resulted in a majority of the conflict's half-million dead.

In retaliation for Wednesday's chemical assault, US President Donald Trump ordered a missile strike on Friday on a Syrian air base housing a jet fleet responsible for extensive bombing across northern Syria.

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The barrage of 59 Tomahawk cruise missiles fired from two US ships is the first direct military action the United States has taken against the Government of Syrian President Bashar al-Assad in the six-year-long conflict. Although Trump warned of possible further intervention, the Pentagon has said no other strikes against government targets are in current plans.

American officials had predicted that the missile strike would result in a major shift in Assad's calculus, but the US attack appeared to be symbolic in reality. Within 24 hours of the strike, monitoring groups reported that warplanes were again taking off from the bombed Shayrat air base, this time to attack Isis (Islamic State) positions.

There were also reports of Syrian Government and Russian airstrikes across the provinces of Damascus, Aleppo, Idlib and Daraa, all killing civilians. There were no reports of further use of chemical weapons.

"The American strikes did nothing for us. They can still commit massacres at any time," said Majed Khattab, speaking by phone from Khan Sheikhoun. "No one here can sleep properly; people are really afraid."

Turkish Foreign Minister Mevlut Cavusoglu described Trump's decision to retaliate as welcome but not enough. "If this intervention is limited only to an air base, if it does not continue, and if we don't remove the regime from heading Syria, then this would remain a cosmetic intervention," he said.

Trump defended the operation against criticism from some members of Congress and military analysts that the volley of missiles did not target the runways. Administration officials have said the attack successfully destroyed refuelling stations, hangars and some planes.

"The reason you don't generally hit runways is that they are easy and inexpensive to quickly fix (fill in and top)!" Trump wrote on Twitter from Mar-a-Lago after playing a round of golf. In an earlier message, Trump offered: "Congratulations to our great military men and women for representing the United States, and the world, so well in the Syria attack."