Douma was the last rebel-held town in the Eastern Ghouta suburb of Damascus, and is the site of the alleged gas attack that prompted a volley of Western air strikes on Saturday.

Rebels had captured it in 2012, and Syrian government fighters had not entered it since.

“Units from the internal security forces entered Douma in Eastern Ghouta,” state television reported.

“We are hours away from ending the terrorist presence in Douma,” it added.

The Syrian Observatory for Human Rights, a Britain-based monitor, said the units had entered after the last buses of opposition fighters left Douma on Saturday for northern Syria.

The departures come under a negotiated withdrawal reached last weekend between the Jaish al-Islam Islamist rebel faction, Syria’s regime, and its Russian ally.

Under the deal, Russian military police were to deploy in the town but Syrian forces were not to enter.

Jaish al-Islam has said it only agreed to the deal because of the Syrian government’s purported use of toxic chemicals on Douma, which medics said killed more than 40 people.

Syria and Russia have both denied using chemical weapons and said the claims were fabrications used to justify Western military action.

A team from the global chemical watchdog was meant to start an investigation inside Douma on Saturday.

Syria said the US, French, and British strikes were an attempt to “hinder the mission’s work”.