Hillary Clinton Wednesday bade farewell to her NATO allies after making her last appearance as US Secretary of State here, praising the military alliance for having made "great strides".

"I've spent a bit of my time in this building over the past four years, and I think it was time well spent," Clinton mused, speaking after two days of talks among the 28-member alliance in Brussels.

"The alliance has made great strides," the top US diplomat said, praising the 60-year old organisation, founded in the early days of the Cold War, as one of the world's "greatest forces for stability and security".

Clinton has been a regular visitor to the rambling NATO headquarters over the four years of her tenure, attending its six-monthly foreign ministerial meetings.

But she is due to step down early in 2013, saying she wants to return to private life after two decades in the public eye.

Rumours are rife in Washington about her potential successor, with US ambassador to the United Nations Susan Rice and veteran Democratic senator John Kerry the odds-on favourites to head up America's diplomacy.

But Rice's long-held hopes of getting the top job are now hostage to Washington power games as Republicans gun for the UN envoy, claiming she misled the American people over the attack on the US mission in Benghazi, Libya on September 11.

Clinton ran through a list of the alliance's achievements, including planning the draw-down of the NATO-led operation in Afghanistan, "a major successful operation in Libya", resuming deep-frozen talks with Russia and the enlargement of the alliance.

The United States is "grateful" to NATO, Clinton said, insisting that the alliance is now "needed more than ever and so we must all continue to invest in it.

"After more than 60 years it keeps us safe, it projects security and stability globally. And through our partnerships we are able to do more in more places," Clinton said.

"For the United States we find it extremely valuable to be able to consult closely with our European allies on challenges from Syria to the Middle East and North Korea."

The United States, Canada and 10 European allies signed a treaty in Washington on April 4, 1949 creating the mutual defence alliance based on solidarity against threats from the Soviet Union.

The United States is by far the biggest contributor, accounting for 75 percent of total NATO defence spending, compared to 50 percent a decade ago.

The alliance's central tenet is Article 5, which states that an attack on one NATO nation represents an attack on all.

On Tuesday, NATO ministers agreed to deploy US-made Patriot missiles along Turkey's border with Syria following a series of cross-border artillery attacks.

The decision came amid growing fears the regime of President Bashar al-Assad could resort to using chemical weapons as it battles a 21-month rebellion.

Clinton told journalists that the US "expects to make a contribution to this essential NATO mission."

The United States had sent a clear message that any use of chemical weapons "would cross a red line and those responsible would be held to account," she added.

During her series of NATO meetings, Clinton has also been feted and given a warm farewell by her counterparts.

On Tuesday at the start of her visit, NATO Secretary General Anders Fogh Rasmussen told her "it is sad" that it was her last trip to the alliance's HQ.

And she was also presented with the Bulgarian foreign ministry's top award, the Golden Laurel, by Foreign Minister Nikolay Mladenov, who thanked her for America's friendship with his country.