Syria's opposition chief said on Wednesday he was "surprised" by a US decision to reject his demand for NATO to provide Patriot missile protection for rebel bastions in the country's north.

"There is an international will that the revolution does not come out victorious," Ahmed Moaz al-Khatib, head of the National Coalition, said in Doha, Qatar.

"But the people that have defied injustice and tyranny will not stop," said Khatib, who still acts as the head of the coalition despite announcing his resignation on Sunday.

He told an Arab League summit on Tuesday that he had asked US Secretary of State John Kerry to extend the umbrella provided by Patriot anti-missile batteries positioned in Turkey to protect rebel-held parts of north Syria.

Khatib said he was waiting for a NATO response, in an address to Arab leaders after the League gave the opponents of President Bashar al-Assad the seat of Syria at the 22-member organisation.

But the White House reacted swiftly saying that "at this time, NATO does not intend to intervene militarily in Syria".

"I think that a Patriot missile battery would follow the definition of military assistance," White House spokesman Jay Carney said, adding that Patriot anti-missile batteries in Turkey were for self-defence only.

In Brussels, NATO chief Anders Fogh Rasmussen on Wednesday echoed the White House's position and said the alliance has "no plans to change the purpose of the coverage of the deployed Patriot missiles", despite the opposition plea.

"We made clear from the outset that this deployment is purely defensive. We have no offensive intentions. We are there to ensure effective defence and protections of Turkish people and territory," Rasmussen told reporters.

"That's a core task of NATO to protect our allies and we stick to that declaration that was issued when we decided to deploy."