Gunmen have robbed Sri Lanka's only McDonald's restaurant, making off with more than $13,000 in cash after locking the staff in a freezer room.

The restaurant in Colombo was closed and staff were counting cash late on Sunday when the robbers entered through a back door used to put out the rubbish.

The six robbers, armed with pistols, then ordered the staff into the freezer room.

"The fast food staff spent about two hours trying to get out of cold storage," a police spokesman said.

Search underway

The spokesman said a major search has been launched for the gunmen.

Fingerprint experts and sniffer dogs have been brought in - but police have yet to make an arrest.

The robbery took place in a high security area of the capital and near the official residence of the parliamentary speaker.

The McDonald's is also located less than half a kilometre away from military checkpoints along Galle Road in Colombo's fashionable Kollupitiya area.