The US, UK and France have launched air strikes against what they allege are Syrian chemical weapons facilities in response to chemical weapons attack in a Damascus suburb a week ago.

The Pentagon said the air strikes, which began at 4am Syrian time (2am GMT), involved planes and ship-launched missiles, more than 100 projectiles in all. Officials named three targets: a scientific research centre in Damascus, a chemical weapons storage facility west of Homs, and another storage site and command post nearby. “Right now, we have no additional attacks planned” the US defence secretary, James Mattis, said. “This is a one-time shot.”

However, in a televised address from the White House earlier to announce the strikes, Donald Trump said the US and its allies would strike again if there were more chemical weapons attacks by the regime of Bashar al-Assad, as reported by The Guardian.

“We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” he said. Referring to last Saturday’s chemical weapons attack reported to have killed over 70 people, Trump said. “These are not actions of a man, they are crimes of a monster instead.”

After Trump finished his seven-minute address, Theresa May and Emmanuel Macron made separate announcements of British and French participation, stressing that the strikes were limited to Syrian regime chemical facilities, and had no wider goals. May said there was no alternative to the action the three countries were taking.

Explosions were reported in Damascus moments after Trump’s address. Later a Syrian official said all sites had been evacuated “days ago” after a warning from Russia.