BEIRUT, Lebanon — A United Nations security team was fired on in the Syrian town of Douma, raising doubts on Wednesday about when international investigators would be able to visit the site of what was suspected to have been a chemical attack.

Eleven days have passed since rescue and medical workers in Douma reported an attack that left victims choking and foaming at the mouth, killing at least 43 people.

Blaming the government of President Bashar al-Assad for the attack, the United States, Britain and France launched coordinated strikes on Syrian military facilities on Saturday, describing them as punishment for what they called Mr. Assad’s use of chemical weapons.

The Syrian government and its Russian and Iranian allies have denied that any chemical weapons were used and blamed rescue workers and rebels for what they called concocted videos said to show the suffering of victims as a way of demonizing the Syrian government.