Turkey has asked NATO to extend for another year the deployment of surface-to-air Patriot missiles to protect its troubled border with Syria because of a continuing "serious" threat, officials said on Wednesday.

"We have received a letter from the Turkish government requesting the continuation of the Patriot mission," a NATO official told AFP on condition of anonymity.

"The North Atlantic Council has regularly assessed the situation and the implementation of the Patriot mission. It is clear that the overall risks and threats to Turkey remain serious," the official said.

A Turkish foreign ministry diplomat, contacted by AFP, also confirmed Ankara's request.

Turkey turned to its NATO allies after a mortar bomb fired from Syrian territory killed five Turkish civilians in the border town of Akcakale in October last year.

Since the deadly attack, Turkey has retaliated in kind for every Syrian shell that has landed on its soil and beefed up its volatile 910-kilometre (560 mile) frontier.

Washington confirmed that Turkey has asked NATO "to continue to augment its air defence capabilities to aid in the defence of its population and territory."

"We look forward to working through NATO to address the request," State Department spokeswoman Jen Psaki said, adding that "we respect and value Turkey as a long-standing NATO and US ally."

Without specifying whether Washington would agree to the demand, Psaki added the US did "recognise the needs they have."

The six batteries of the US-made missiles, effective against aircraft and short-range missiles and dispatched by the Netherlands, Germany and the United States, are deployed in the southern city of Adana and the southeastern cities of Kahramanmaras and Gaziantep.

"Allies have shown a strong commitment to protect and defend Turkey," said the NATO official.

"Any continuation of the deployment would reaffirm the determination of NATO to deter threats and defend Turkey, reflecting and confirming once again NATO's solidarity with Turkey."

NATO approved their initial deployment in December, saying the use of ballistic missiles by the Syrian regime posed a threat to Turkey.

But Syria's allies Iran and Russia opposed the Patriot deployment, fearing that it could spark a regional conflict also drawing in NATO.

Originally used as an anti-aircraft missile, Patriots today are used to defend airspace by detecting and destroying incoming missiles. They were made famous during the 1991 Gulf War as a defence from Scuds fired on Israel and Saudi Arabia from Saddam Hussein's Iraq.

NATO deployed Patriot missiles in Turkey during the 1991 Gulf war and in 2003 during the Iraqi conflict.

Turkey was once a friend and ally of Syrian President Bashar al-Assad but relations have broken down since the Syrian conflict began in 2011, sending more than 600,000 refugees across the border.

Ankara has backed the opposition fighting to topple the embattled leader.