Syria dominated US Secretary of State John Kerry's first NATO foreign ministers meeting Tuesday as reports of chemical weapons use by Damascus threatened to raise the stakes dramatically.

As fighting intensifies, ministers were increasingly concerned by the prospect of the bloody conflict spilling over into an already tense region which is home to one of the alliance's key members, Turkey.

Ministers also discussed NATO's planned withdrawal from Afghanistan in 2014, winding down a campaign begun in 2011 in the aftermath of the 9/11 terror attacks which were echoed in last week's bombing of the Boston marathon.

"We can all see that the situation in Syria is getting worse," NATO head Anders Fogh Rasmussen said on arriving for the talks.

"We cannot ignore the risks of regional spillover, with (the) possible implications for Allied security," Rasmussen said.

Kerry made a similar point earlier but on Tuesday, the issue of chemical weapons -- which Washington has warned would be a red line in the conflict -- came centre stage after Israeli army officials said they had been used.

"To the best of our professional understanding, the (Assad) regime has made use of deadly chemical weapons against the rebels in a number of incidents in the last few months," Israeli Brigadier General Itai Brun, head of the research and analysis division of military intelligence, told a forum.

Asked about the reports, Kerry said he had telephoned Israeli Prime Minister Benjamin Netanyahu who had been unable to confirm the information.

"I think it is fair to say (the prime minister) was not in a position to confirm that in the conversation," Kerry said, adding: "I do not know yet what the facts are."

Kerry, plainly aware of the sensitivity of the issue, said that allegations made about the use of chemical weapons "have to be thoroughly investigated."

Washington has warned of the danger of chemical weapons being used by President Bashar al-Assad or of their falling into the hands of groups like Lebanon's Hezbollah which is backed by Iran and whose fighters are in action against the rebels.

Kerry told his colleagues that NATO must be aware of the threats from Syria.

Stressing that he was not calling for any active planning, Kerry said "we should ... carefully and collectively consider how NATO is prepared to respond to protect its members from a Syrian threat, including any potential chemical weapons threat."

NATO has no direct role in the conflict but it has deployed Patriot anti-missile batteries along member Turkey's border with Syria to prevent any spillover of the fighting, Kerry noted.

On Afghanistan, Rasmussen said NATO was making good progress in handing over frontline security duties to Afghan forces, pledging that the alliance would stay the course through 2014 and into its new training mission afterwards.

Kerry is to hold a meeting in Brussels Wednesday with Afghan President Hamid Karzai and the head of Pakistan's armed forces, Ashfaq Kayani and other top Pakistani officials with the aim of easing tensions between the two neighbours.

Relations between Islamabad and Kabul have been strained for many years and Karzai has accused Pakistan of aiding the Taliban, who seek refuge along their rugged border.

"We ... need the positive engagement of Afghanistan's neighbours, including Pakistan," Rasmussen said.

Kerry also had a one-on-one meeting with his Russian counterpart Sergei Lavrov, largely devoted to the deteriorating situation in Syria and the need for a political solution, a US official said.

Moscow favours a much less interventionist approach in Syria, a longstanding Russian ally, and Lavrov made that point again at a press conference.

Recent developments "show that there is a growing understanding of the real threats we will all face" if dialogue is blocked by a "minority of the international community," said Lavrov.

Lavrov attended the meeting as part of regular NATO-Russia consultations on a bilateral and international issues.