A majority of lawmakers ratified the Mutual Defence Cooperation Agreement (MDCA) as hundreds of Communist, left-wing and anti-war demonstrators protested on central Syntagma Square.

The agreement, which updates a deal originally signed in 1990 when Prime Minister Kyriakos Mitsotakis' father was premier, enables the use by US forces of Greek military facilities including the airbases of Larissa and Stefanovikio.

Permitted actions for US forces at these facilities include training, refuelling, temporary maintenance, storage and emergency response.

The updated deal was originally signed in October during a visit by US Secretary of State Mike Pompeo.

"This agreement is beneficial to national interests," defence minister Nikos Panagiotopoulos told parliament.

The US is also granted priority status for "unimpeded access and use" within 48 hours of the port of Alexandroupolis, a Balkans and Black Sea gateway of strategic value to the US navy and NATO.

The US paid approximately $2.3 million to remove last year a sunken dredging barge that had blocked part of the harbour since 2010.

"This agreement upgrades (Greece's) importance in US planning and can help deterrence," the defence minister said, adding that US forces are expected to invest over 12 million euros ($13 million) on the Larissa base and around six million euros at Marathi, part of the US base of Souda, Crete.

Athens has signed on to an F-16 upgrade programme worth $1.5 billion, and is also interested in US-made drones and, eventually, F-35 warplanes, Mitsotakis has said.

The defence agreement has been roundly condemned by left-wing opposition parties.

"Wherever they are, US and NATO bases are targets for reprisals...NATO bases do not protect," said Communist party chairman Dimitris Koutsoumbas, warning that the government was "deepening" its involvement in "adventurism."