Beirut (AFP) - Two warplanes took off from a central Syrian airbase Friday hours after it was struck by US missiles and carried out bombing raids nearby, a monitoring group said.

The Syrian Observatory for Human Rights said the aircraft "took off from inside the Shayrat base, which is partially back in service, and struck targets near Palmyra".

Observatory head Rami Abdel Rahman could not specify whether they were Syrian or Russian planes, but said they were Sukhoi jets, which both Damascus and its ally Moscow use.

The Britain-based group said the aircraft targeted territory controlled by the Islamic State jihadist group, which holds parts of the central Syrian province of Homs.

Early on Friday morning, the US military fired 59 Tomahawk missiles at the Shayrat air field in response to a suspected chemical attack this week that has been widely blamed on the Damascus regime.

A Syrian military source told AFP that Syria's armed forces were warned about possible US military action hours before the strike took place.

"We took precautions in more than one military point, including in the Shayrat airbase. We moved a number of airplanes towards other areas," the source said.

US officials said Russia's military in Syria had been informed of the strike beforehand in order to avoid casualties that could prompt a broader crisis.

The US said the missiles targeted radars, aircraft, and air defence systems and destroyed around 20 Syrian planes, but said the runway was intact.

Russia's military said the strike had an "extremely low" military impact, with fewer than half of the 59 missiles reaching the airbase.

According to the Observatory, the US strike on the base killed eight members of Syria's armed forces, including a doctor.

Syria's army had earlier said six people were killed in the strike, without specifying if they were military personnel or civilians.

The official state news agency SANA also the strikes also killed nine civilians in villages near the base.