Israel's willingness to hit Syrian targets it sees as threats to its own existence has complicated the Obama administration's internal debate over arming President Bashar Assad's foes and may change the way US approaches allies as it tries to boost the rebels, including with possible military aid.
As secretary of state John Kerry flew to Russia on Monday for talks with the Assad regime's most powerful ally, the administration remained tight-lipped on both Israel's weekend air strikes and their implications for Washington decision-making.
Israeli warplanes targeted caches of Iranian missiles that were bound for Hezbollah, the Lebanon-based militant group that has threatened Israel. The weapons would have allowed Hezbollah to strike Tel Aviv and as far as southern Israel from inside Lebanese territory.
Still, Israel's actions put Damascus and Moscow on notice that the U.S. and its allies may not wait for an international green light to become more actively engaged.
The administration said last week it was rethinking its opposition to arming the rebels or taking other aggressive steps to turn the tide of the two-year-old civil war toward the rebels.
At the same time, Israeli involvement in the war carries risks. Instead of prodding Russia into calling for Assad's ouster, it could bring greater Arab sympathy for Assad and prompt deeper involvement from Iran and the Lebanese militant group Hezbollah, actors committed as much to preserving Assad as to fighting the Jewish state.
Although Israel has not officially acknowledged it carried out the airstrikes, Syrian officials on Monday were blaming Israel, calling it a "declaration of war" that would cause the Jewish state to "suffer."
Russia, alongside China, has blocked U.S.-led efforts three times at the United Nations to pressure Assad into stepping down. Officials said Kerry hopes to change Moscow's thinking with two new arguments: American threats to arm the Syrian rebels and evidence of chemical weapon attacks by the Assad regime.
Kerry, US officials said Monday, hopes that may be enough to persuade Russian President Vladimir Putin to support, or at least not veto, a fresh effort to impose UN sanctions on Syria if Assad doesn't begin transition talks with the opposition.
The officials demanded anonymity because they weren't authorized to speak publicly about the confidential diplomacy.
"We have consistently, in our conversations with the Russians and others, pointed clearly to Assad's behavior as proof that further support for the regime is not in the interest of the Syrian people or in the interest of the countries that have in the past supported Assad," White House spokesman Jay Carney said.
"We have been clear in the past about our disappointment with Russia over their opposition to resolutions at the Security Council with regards to this matter. But this is an ongoing conversation," he said.
US officials said the administration doesn't believe the weekend activity will force President Barack Obama's hand, noting that the US's main concern is the use of chemical weapons by Assad, while Israel's top concern is conventional weapons falling into the hands of its enemies.
The chemical weapons argument is now under surprising attack, with former war crimes prosecutor Carla Del Ponte saying over the weekend she and fellow members of a four-member UN human rights panel have indications the nerve agent sarin was used by Syrian rebel forces, but not by government forces.
Despite a clarification from the United Nations that it is has not yet made any definitive determination on chemical weapons use, Washington pushed back on del Ponte's assertion, saying it's highly likely that the Assad regime, and not the rebels, has been behind any chemical weapons use in Syria.
"We are highly skeptical of suggestions that the opposition could have or did use chemical weapons," Carney said.
"We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime. And that remains our position."
The state department said the administration continues to believe that Syria's large chemical weapons stockpiles remain securely in the regime's control.
The Obama administration opened the door to new military options in Syria after declaring last week it strongly believed the Assad regime used chemical weapons in two attacks in March.
Two days after that announcement, defence secretary Chuck Hagel said arming the Syrian rebels was a policy consideration.
Before departing for Russia, Kerry visited the Pentagon for a lunch meeting with Hagel. Defence department press secretary George Little said he expected Syria to be discussed.
Also on Monday, the chairman of the senate foreign relations committee called for the United States to provide weapons to vetted Syrian rebels.
Senator Bob Menendez, a Democrat, introduced legislation that would allow for arms, military training and non-lethal aid to rebels that meet certain criteria on human rights and don't have links to terrorism.
Until now, US efforts to bolster the rebels' fighting skills and gather intelligence on the groups operating inside Syria have been limited to small training camps in Jordan, according to two US officials, who weren't authorized to speak about secret activities and demanded anonymity.
There are several options for escalation ranging from arming the rebels to targeted airstrikes and imposing no-fly zones. However, arming the rebels is the most likely escalation, officials said.
Officials said targeted strikes are likely to be considered only after uncontested proof emerges of chemical weapons use.
And, even the most ardent advocates of US intervention don't want American military boots on the ground while no-fly zones would demand intensive operations to neutralize Syria's Russian-supplied air defenses.
Although Israel seems to have thwarted those defenses with its weekend strikes, US officials say that maintaining permanent no-fly zones will require far more support than specific actions like the airstrikes.
After visiting Moscow for the first time since he became secretary of state, Kerry will travel to Rome for talks with members of the new Italian government as well as Israeli justice minister Tzipi Livni and Jordanian Foreign Minister Nasser Judeh to discuss Middle East peace prospects.