Fears resurfaced Tuesday of Syrian President Bashar al-Assad resorting to chemical weapons as rebels battled regime and Hezbollah forces in fierce fighting in Homs province.
In Brussels, US Secretary of State John Kerry told foreign ministers from the 28 NATO nations that the alliance must stand prepared for threats from Syria, including the possible use of chemical weapons which Washington has said would be a red line, drastically changing the issues at stake.
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".
He later said that Israel Prime Minister Benjamin Netanyahu had not been able to confirm to him Tuesday that the Assad regime had used chemical weapons against rebels, as reported by a senior Israeli intelligence official.
"I think it is fair to say (the prime minister) was not in a position to confirm that in the conversation," Kerry said, explaining that he had telephoned Netanyahu following reports of their use.
"I do not know yet what the facts are," he told a press conference.
The White House meanwhile reinforced the point, saying it was monitoring the situation and had not come to a conclusion.
"We support an investigation, we're monitoring this, and we have not come to the conclusion that there has been that use" of chemical weapons, White House spokesman Jay Carney said.
Carney recalled that for President Barack Obama, the use of chemical weapons would be "unacceptable."
"There are those in the Syrian government who have expressed a willingness to use chemical weapons to protect their interests and prolong the rule of the Assad regime," the spokesman said.
"We remain skeptical of any claim that the opposition used chemical weapons."
Earlier, the head of research and analysis in the Israeli army's military intelligence division, Brigadier General Itai Brun, said Assad was indeed guilty of using chemical weapons, likely sarin, against rebel fighters.
The war in Syria and the possible use of chemical agents also topped the agenda in Pentagon chief Chuck Hagel's talks with Jordan's Prince Faisal and army chief General Masbal al-Zaben, officials said.
Meanwhile, two Orthodox bishops reportedly kidnapped by rebels in northern Syria have been released, a statement from a religious group said.
The Paris-based "Oeuvre d'Orient" association said the two -- Bishop Yohanna Ibrahim and Bishop Boulos Yaziji, who were seized on Monday -- were already at Saint Elias cathedral in Aleppo.
On the battle-front, the Syrian Observatory for Human Rights watchdog and activists said Hezbollah, which is backed by Iran, was fighting alongside Syrian troops in the Qusayr area of the central province of Homs.
A Syrian military source insisted, meanwhile, that the capture of the rebel stronghold town of Qusayr was "just days away, at most".
Hezbollah's role in the fighting has inflamed tensions inside Lebanon, despite its insistence it is only acting to protect Lebanese citizens in Syrian border villages.
Lebanon's Salafist Sunni Sheikh Ahmed al-Assir has urged his followers to join the fight against Syrian regime and Hezbollah forces in Qusayr, calling it "a religious duty".
The conflict, which began in March 2011 and has killed more than 70,000 people, has regularly spilled across the border, with two new mortar rounds hitting the eastern Lebanese region of Hermel on Tuesday.
In letters to the United Nations, meanwhile, Damascus labelled as an "act of aggression" an EU decision to ease an oil embargo on Syria, in a move aimed at allowing rebels exploit the resources they control.
At least 134 people were killed throughout Syria on Monday, according to the Observatory, including 50 rebels, 47 civilians and 38 soldiers.