– Israel declared Tuesday that it had found evidence that the Syrian government had repeatedly used chemical weapons last month, arguing that President Bashar Assad was testing how the United States and others would react and that it was time for Washington to overcome its deep reluctance to intervene in the Syrian civil war.

Brig. Gen. Itai Brun, Israel's top military intelligence analyst, said at a security conference in Tel Aviv on Tuesday that Syria used chemical weapons, probably a sarin-based nerve agent, in attacks March 19 near Aleppo and Damascus. He said the assessment was based on reports of victims foaming at the mouth and having constricted pupils.

In making the declaration — which went somewhat beyond recent suspicions expressed by Britain and France — Israeli officials argued that Assad had repeatedly crossed what President Obama said last summer would be a "red line."

Administration officials pushed back, however, saying they would not leap into the conflict on what they viewed as inconclusive evidence, even while working with allies on plans to secure the weapons if it appeared they were about to be used or handed to Hezbollah.

Officials said that an investigation was necessary but added that U.S. intelligence agencies had yet to uncover convincing evidence that an attack on March 19 — and smaller subsequent attacks — used sarin gas, a deadly agent that Syria is believed to hold in huge stockpiles.

"We are looking for conclusive evidence, if it exists," White House press secretary Jay Carney said.

In a briefing in Tel Aviv, an Israeli military official was vague about the exact nature of the evidence, saying that it was drawn from an examination of photographs of victims and some "direct" findings that he would not specify. U.S. Secretary of State John Kerry suggested there were mixed messages emerging from Israel, saying that he had spoken to Prime Minister Benjamin Netanyahu on Tuesday morning and that the Israeli leader "was not in a position to confirm" the intelligence assessment. Israeli officials said they would not attempt to explain the apparent difference between Netanyahu's statement and that of his top military intelligence officials.

At the same time, Daniel Shapiro, U.S. ambassador to Israel, said that contingency plans to address the use of chemical weapons in Syria were "very much part" of the discussions between Defense Secretary Chuck Hagel and his Israeli counterpart in Tel Aviv on Monday.

Brun told participants at a security conference in Tel Aviv that the Syrian government "has increasingly used chemical weapons." That echoed accusations that Britain and France made in a letter last week to the secretary-general of the United Nations, calling for a deeper investigation.

Brun's statements were the most definitive to date by an Israeli official regarding evidence of possible chemical weapons attacks March 19 near Aleppo and Damascus, the capital.

Another military official, speaking on the condition of anonymity, said that the evidence had been presented to the Obama administration but that Washington had not fully accepted the analysis.

No physical proof

None of the assertions — by Israel, Britain or France — included physical proof. Experts say the most definitive way to prove the use of chemical weapons is to promptly collect soil samples at the site and examine suspected victims.

A senior Defense Department official noted that "the use of chemical weapons in an environment like Syria is very difficult to confirm." He added: "Given the stakes involved, low-confidence assessments by foreign governments cannot be the basis for U.S. action. … Thus, we must be absolutely confident of use before determining how to respond."

That will not be easy. The Syrian government, which has accused insurgents of using chemical weapons and has requested that a U.N. forensics team investigate, has refused to allow that team to enter the country because of a dispute over the scope of its inquiry.

Charges of chemical weapons use come as the United States and its allies struggle to find a viable approach to the Syrian conflict, now in its third year.

The White House has been trying to slowly build pressure on Syrian President Bashar Assad, while stopping short of providing military aid to the rebels. Over the weekend, officials pledged an additional $123 million in nonlethal aid.

The Tribune Washington Bureau contributed to this report.