Intelligence officials around the world were investigating the accusations, U.S. officials told CNN on Wednesday.
Investigators were talking to rebels and defectors, poring over medical intelligence regarding symptoms reported by doctors and looking at satellite imagery used to track missiles launched and chemical weapons movements, the officials said on condition of anonymity. They were not authorized to release details to the media.
A spokesman for Netanyahu, Mark Regev, told CNN that Israeli officials had no confirmation that chemical weapons had been used.
But Regev's comments did not square with those of Israeli Justice Minister Tzipi Livni.
"It is clear for us here in Israel" that chemical weapons have been used in Syria, and an international response should be on the table, Livni told CNN in an exclusive interview from her home in Tel Aviv.
Livni wouldn't say whether there is evidence the Syrian government has directed the use of any chemical weapons.
But she said the development would pose a direct threat to Israel, which shares a border with Syria.
Their concerns centered on an attack Tuesday in Khan al-Asal in the northern province of Aleppo. State-run media blamed rebels for the attack, which it said killed 25 people and injured more than 110 others.
Rebels say they have no chemical weapons
On Wednesday, Syria's U.N. ambassador, Bashar Ja'afari, said the Syrian government has asked Secretary-General Ban Ki-moon to open an investigation into the alleged use of chemical weapons by "terrorist groups," which is how the government refers to rebels.
But the opposition Free Syrian Army said rebels don't have access to chemical weapons and blamed the casualties on a government missile.
Separately, an opposition group said the government attacked the rural Damascus suburb of Ateibeh with "chemical rockets," causing an unspecified number of deaths along with cases of suffocation, nausea and hysteria. There was no immediate government response.
The reports ignited a firestorm of reactions, with Russia slamming the rebels and some U.S. lawmakers saying that Washington might need to take action against the Syrian government.
Observers: Images are not consistent with a chemical weapons attack
But images posted by Syrian state-run media of the aftermath of the Aleppo incident, which the government blamed on rebels, are not consistent with a chemical weapon attack, some observers said.
"There are no images of the site of the attack; just of some affected people. These people do not show outward symptoms of a CW (chemical weapon) attack. Definitely not mustard; definitely not a nerve agent," wrote Jean Pascal Zanders, senior research fellow at the European Union Institute for Security Studies.
"There are far too many people, including non-medical staff, around the affected persons. Apart from a surgical mask, nobody wears any protective garment or gas masks. If there would have been a CW attack with one of the agents known (or believed) to be in Syria's arsenal, then most of the people present would have been fatally or seriously contaminated."
He added that, during the Arab uprisings, witness reports cited chemical attacks, but none had been confirmed. "People are exposed to a wide range of toxicants in today's battlefield," he said in an e-mail. "Furthermore, once a rumor gets around, people are more likely to think that they suffer from symptoms similar to the ones being rumored."
Not the first round of claims
U.S. officials pointed to previous claims that the Syrian government used chemical weapons, which were found to be false after extensive investigation.
The Syrian government did not use chemical weapons against residents of Homs in a December attack, a U.S. State Department investigation showed, but did apparently misuse a riot-control gas in the incident, according to senior U.S. officials.
The officials said the State Department launched a probe from its consulate in Istanbul, Turkey, after reports from doctors and activists that dozens of people suffered nervous system, respiratory and gastrointestinal ailments after inhaling the gas in Homs on December 23.
The civil war -- which began two years ago after a government crackdown on Syrian protesters -- has left around 70,000 people dead and uprooted more than 1 million others, the United Nations has said.