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“The regime has increasingly used chemical weapons,” said Brig. Gen. Itai Brun, research commander in the intelligence directorate of the Israeli Defense Forces, echoing a recent finding by Britain and France. “The very fact that they have used chemical weapons without any appropriate reaction,” he added, “is a very worrying development, because it might signal that this is legitimate.”

General Brun’s statements are the most definitive by an Israeli official to date regarding evidence of chemical weapons attacks on March 19 near Aleppo and Damascus. Another military official, speaking on the condition of anonymity, said that the evidence had been presented to the Obama administration — which has declared the use of chemicals a “red line” that could prompt American action in Syria — but that Washington has not fully accepted the analysis.

“When you draw a red line, you have very little interest in crossing it, if crossing it means you have to take action,” said the official, who was not authorized to address the matter publicly.

In briefings on Tuesday, the Israelis said they believed that the attacks March 19 involved the use of sarin gas, the same agent used in a 1995 attack in the Tokyo subway that killed 13. The Syrian attacks killed “a couple of dozens,” the military official said, in what Israel judged as “a test” by President Bashar al-Assad of the international community’s response. He said the government had deployed chemicals a handful of times since, but that details of those attacks were sketchier.

“Their fear of using it is much lower than before using it,” the official said. “If somebody would take any reaction, maybe it would deter them from using it again.” Regarding possible further attacks, he added, “Now I’m more worried than I was before.”

Israel has been deeply reluctant to act on its own in Syria, for fear that it could bolster President Assad by uniting anti-Israel sentiment. But the public statements regarding the attacks, days after the British and French governments wrote to the United Nations Secretary General saying they, too, had evidence of chemical use, complicates the situation for Washington.

President Obama said last month during his visit to Israel that proof of chemical weapons use would be a “game changer.” But Defense Secretary Chuck Hagel said Monday that the intelligence regarding the attacks remained inconclusive, and his press secretary, George Little, said Tuesday that the Pentagon was continuing to assess reports on the matter.

“The use of such weapons would be entirely unacceptable,” Mr. Little said in Amman, Jordan, where Mr. Hagel landed Tuesday. “We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah.”

Speaking about Syria at a conference of Israel’s Institute for National Security Studies here, General Brun said “it is quite clear that they used harmful chemical weapons,” citing “different signs” including pictures of victims “foaming at the mouth.” He went beyond the March 19 attack to speak of “continuous” use of such weapons, and described a “huge arsenal” of more than 1,000 tons stockpiled in Syria.

The military official who spoke on the condition of anonymity said that Israel based its analysis mainly on publicly available photographs of victims, but said there was also corroborating “direct evidence” that he would not detail.

The United States has also made efforts to gather evidence from the field. 

 Majid, a rebel commander from the eastern suburbs of Damascus, said his battalion had been contacted, through intermediaries, by the Central Intelligence Agency, requesting samples to be tested for the presence of chemicals. Speaking via Skype from Jordan, and on the condition he be identified only by first name for his safety, Majid said the American intelligence agency had requested soil, urine and hair samples from several areas around Damascus: Jobar, a northeastern neighborhood of the city that has been fiercely contested in recent months; Adra, an industrial area north of the city; and Ataibeh, northeast of the capital. 

“We’re still waiting to get the samples,” Majid said, explaining that it would take time because of the difficulty of traveling to contested areas.

Louay Mekdad, a spokesman for the Free Syrian Army, has also said the umbrella group of rebels would collect evidence of the attacks, including testimony from doctors and patients and physical samples.

Though the Assad government had claimed last month that it was the rebels who used chemicals, General Brun echoed previous statements by Israeli and American officials that it was clearly the Syrian government, and not the opposition, that had conducted the attacks.

Israeli military officials said that over the past few months Syria has sharply consolidated its chemical stockpiles, reducing the number of sites by about half to retain greater control over the arsenal. The weapons are now stored in 15 to 20 sites, they said.

If American officials have been more reluctant that their allies to come to firm conclusions, it may be because it would force Mr. Obama’s hand. In August, the president told reporters that any evidence that Mr. Assad was moving the weapons or making use of them could prompt the United States to act.

“That would change my calculus,” he said. “That would change my equation.”

But when strong evidence emerged earlier this year that Mr. Assad’s forces were in fact moving their weapons — at least from one depot to another — the White House insisted that the action did not cross the line that Mr. Obama set. By “move” the weapons, a White House spokesman said, Mr. Obama meant transferring them to a terror group, like Hezbollah.

Nonetheless, according to two American officials, Washington sent messages to President Assad that the threat had to be taken seriously. “We saw a reaction,” one official said. Protection of the sites was improved. While the United States has drawn up plans to seize control of the weapons if need be — by parachuting in troops to the key sites — American officials have made it clear that they would prefer that regional forces take the lead. But if the weapons were actually used, as three American allies now contend, it would be far more difficult for Mr. Obama to argue that his “red line” has not been crossed.

Israel, which in January bombed a convoy of sophisticated antiaircraft weapons it feared was being transferred from Syria to Hezbollah in Lebanon, has been preparing its own plans, though it far prefers a broader international intervention.

“There is a risk that if Israel will do something there will be no international community or coalition,” said the Israeli military official. “Maybe because Israel is so close Israel sees it differently from the rest of the world. Just imagine if there was a use of chemical weapons in Mexico. Everyone in the southern United States would be very worried about that.”

The Syrian government has never publicly acknowledged that it has chemical weapons, stating simply that it would never use chemical weapons, if it had them, against its own people. But in July, a prominent government spokesman, Jihad Makdissi, raised eyebrows earlier in the conflict by saying that Syria would use chemical weapons only against a foreign attacker, not against its own people. But he also noted that Syria was facing external enemies as part of the conflict.

Some read his wording as an admission that Syria had the weapons. Others noted that since Syria’s government has characterized its armed opponents as foreign and foreign-inspired terrorists, the statement might be laying the groundwork to justify using the weapons against the uprising.

Mr. Makdissi later took a less prominent role and fled the country five months later.