'Huge' explosion near Damascus Airport: monitor

"The blast was huge and could be heard in Damascus," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

The Damascus International Airport lies about 25 kilometres southeast of Syria's capital.

The Britain-based Observatory, which gathers information from sources across war-ravaged Syria, said it had confirmed the explosion had not taken place inside the airport itself.

"It's unclear what caused the explosion, but there fires raging at the site," Abdel Rahman told AFP.

More than 320,000 people have been killed since Syria's conflict erupted in 2011 with widespread anti-government protests.

While Damascus has remained relatively insulated from the violence, fierce fighting has raged on the outskirts of the capital in recent months between rebel groups and government forces.