A US missile strike on an airbase in central Syria early on Friday was "foolish and irresponsible," President Bashar al-Assad's office said.
"What America did is nothing but foolish and irresponsible behaviour, which only reveals its short-sightedness and political and military blindness to reality," a statement said.
The US fired a barrage of 59 cruise missiles at the Shayrat base in response to a suspected chemical attack on a rebel-held town on Tuesday widely blamed on the Damascus regime.
The US missile strike targeting an airbase in central Syria killed nine civilians, including four children, on Friday, state news agency SANA reported.
The agency said five of the civilians, among them three children, were killed in the village of Shayrat just outside the base of the same name.
"An American missile also hit the village of Al-Hamrat, which killed four civilians including a child," it added.
"Another seven civilians were wounded when a missile hit homes in Al-Manzul, four kilometres (two and a half miles) away from the Shayrat air base," it added.The Syrian government has categorically denied the accusation, saying it had struck an arms depot belonging to a jihadist group.
Assad's office said the government would redouble its efforts against rebel groups after the US strike -- the first direct military action by Washington against the Damascus regime.
"This aggression has increased Syria's determination to strike these terrorist agents, to continue crushing them and to speed up the pace of work on this, wherever they are on Syrian territory," it said.
"The disgraceful act of targeting a sovereign state's airport demonstrates once again that different administrations do not change deeper policies."
The massive strike -- US President Donald Trump's biggest military decision since taking office -- marked a dramatic escalation in American involvement in Syria's six-year civil war.
It followed days of outrage at images of dead children and victims suffering convulsions from the suspected sarin gas attack on the rebel-held town of Khan Sheikhun.