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The Guardian’s Middle East correspondent reports from Beirut:

The al-Shayrat air base has been central to the Syrian war and to persistent claims of chemical weapons use. It is one of Bashar al-Assad’s main military institutions and has housed allies including Russian troops, Hezbollah and Iraqi militias throughout the war.

Sources in Lebanon who are allied to the regime said senior officers evacuated the base before the airstrikes happened and some commanders were attempting to move their families to the Lebanese capital.

The US strike had been increasingly anticipated from late on Thursday until early Friday morning, when 59 precision guided missiles were launched from two US destroyers in the Mediterranean.

Russia has retained a presence at al-Shayrat, although its most significant force is located at a purpose-built base close to Latakia.

There was no immediate response from Hezbollah, whose forces have been central to turning the tide of the war in Assad’s favour over the past two years, most recently in Hama, where they have helped rebuff an assault near Syria’s fourth city led by jihadists and opposition groups.

Russia has established a comprehensive ground-radar bubble over much of northern Syria, which would have been capable of detecting incoming threats, such as the pre-dawn barrage of missiles.

Before the war, al-Shayrat was one of Assad’s strategic bases. Located near where large stores of sarin were kept in bunkers, it has maintained that role even as most of the stockpiles were surrendered in late 2013. Since then, there have been several claims of sarin use and extensive claims that chlorine has been strapped to war planes and helicopters and dropped on opposition fighters and communities.