White House Press Secretary Jay Carney said Tuesday the administration has made no conclusions on whether or not Syrian President Bashar al-Assad’s forces have used chemical weapons against civilians. (The Washington Post)

The Obama administration expressed caution Tuesday about new claims by Israel that the Syrian government has used chemical weapons against rebels.

U.S. officials said they are still evaluating whether the Syrian regime has employed chemical weapons, a step that President Obama has said could trigger direct U.S. involvement in a civil war that has killed more than 70,000 people.

“We support an investigation. We are monitoring this,” White House press secretary Jay Carney said. “We have not come to the conclusion that there has been that use. But it is something that is of great concern to us, to our partners, and obviously unacceptable, as the president made clear.”

Two senior Israeli military officials asserted Tuesday that forces loyal to Syrian President Bashar al-Assad deployed chemical weapons in several incidents that killed dozens of rebel fighters. The officials told reporters in Israel that their evidence — including photographs that purportedly show victims foaming at the mouth — made them “nearly 100 percent” certain.

“To the best of our professional understanding, the regime used lethal chemical weapons against gunmen in a series of incidents in recent months,” said Brig. Gen. Itai Brun, chief of the research division of Israel’s army intelligence.

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A second senior Israeli military officer, speaking on the condition of anonymity to discuss intelligence matters, said chemical weapons appear to have been used in five cases. He said “dozens” of people were killed in the attacks when a “sarin-type” chemical was dispersed.

Brun’s comments, made at a security conference in Tel Aviv, were the most public claim by Israel that Syria has resorted to chemical weapons, a move that would mark a steep escalation in a brutal civil war that is in its third year. But the Israelis released no hard evidence to back up their claims.

Coming less than a week after France and Britain made similar assertions to the United Nations, the remarks from a close U.S. ally added to pressure on Washington to step up assistance to Syrian rebel forces.

Secretary of State John F. Kerry, who is in Brussels for NATO meetings on Syria, said that he spoke with Israeli Prime Minister Benjamin Netanyahu by phone Tuesday morning and that Netanyahu “was not in a position to confirm” the assessment by Israel’s military. Kerry said further investigation is necessary.

Pentagon spokesman George Little, traveling with Defense Secretary Chuck Hagel in the Middle East, said U.S. officials are still conducting their own assessments. He did not address directly whether the U.S. military agreed with the Israeli assertions.

“We are concerned about reports of potential chemical weapons use, which is precisely why we’ve called for a thorough investigation,” he told reporters. “It’s important that we do whatever we can to monitor, investigate and verify any credible allegations, given the enormous consequences for the Syrian people and given [Obama’s] clear statement that chemical weapons use is unacceptable.”

Little declined to comment on whether Israeli officials shared their findings with their U.S. counterparts during Hagel’s three-day visit. But he said that the U.S. government “remains actively engaged with other countries to underscore our common concern about the security of these weapons” and that it was coordinating “closely with our partners, including the French, British and Israelis.”

A senior U.S. defense official said corroborating claims “in an environment like Syria’s is very difficult.” The official, who spoke on the condition of anonymity to discuss sensitive intelligence, indicated that the administration was reacting more skeptically than Israel, Britain or France.

Israel is chiefly concerned that the Lebanon-based Hezbollah militant group could obtain chemical weapons from Syria and use them against Israel. In recent weeks, Israel has been pushing to consider military intervention to destroy Syria’s extensive chemical stockpile. Israeli or U.S. airstrikes would be the most likely means of attacking the arsenal, a move that Israel acknowledges would be difficult and that carries many downsides.

With the Israeli concern in mind, Obama has warned that the transfer of chemical weapons to “non-state actors,” such as Hezbollah, would be unacceptable. He has never said exactly what the United States would do in response to a proven use or transfer of chemical weapons by the Syrian government.

The administration has approved wider battlefield support for Syrian rebels but has stopped short of sending weapons, fearing that they will go astray or lead to an arms race with Russia and Iran, which supply the Assad regime.

In letters last week to U.N. Secretary General Ban Ki-moon, France and Britain said there is credible evidence that Syria has used chemical weapons on more than one occasion since December. According to senior diplomats and officials briefed on the accounts, the evidence included soil samples and witness interviews that point toward nerve agents used in and around the cities of Aleppo, Homs and Damascus.

In Brussels on Tuesday, Russian Foreign Secretary Sergei Lavrov accused “certain Western members” of the U.N. Security Council of politicizing the investigation of the possible use of chemical weapons in Syria and compared it to the hunt for nuclear weapons in Iraq a decade ago.

Lavrov spoke after discussions on Syria with foreign ministers from NATO’s 28 member nations and a separate closed-door session with Kerry. He said any accusations of chemical weapons use should be investigated by experts.

Booth reported from Tel Aviv. Craig Whitlock in Amman, Jordan, Joel Greenberg in Jerusalem and Karen DeYoung in Brussels contributed to this report.