MOSCOW (AP) -- Secretary of State John Kerry is making his case to Russian President Vladimir Putin for Russia to take a tougher stance on Syria at a time when Israel's weekend air strikes against the beleaguered Mideast nation have added an unpredictable factor to the talks.
Kerry arrived Tuesday in Moscow for talks with the most powerful ally of Syrian President Bashar Assad's regime.
Officials said Kerry hopes to change Moscow's thinking on Syria with two new angles: American threats to arm the Syrian rebels and evidence of chemical weapon attacks by the Assad regime.
Over the weekend, Israeli warplanes targeted what Israel claimed were caches of Iranian missiles bound for Hezbollah, the Lebanon-based terror group. Such weapons would allow Hezbollah to strike Tel Aviv and as far as southern Israel from inside Lebanese territory.
Israel's willingness to hit Syrian targets it sees as threats to its own existence has complicated the Obama administration's internal debate over what to do about Syria.
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 in the Syrian conflict. The administration said last week it was rethinking its opposition to arming the Syrian 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 Hezbollah, actors committed as much to preserving Assad as to fighting the Jewish state.
Although Israel hasn't officially acknowledged it carried out the airstrikes, Syrian officials on Monday were blaming Israel, calling them 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.
U.S. officials are hoping Syria's behavior could shift Russia's stance.
"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.
U.S. officials said the administration doesn't believe the weekend activity will force President Barack Obama's hand, noting that the main U.S. 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 U.N. human rights panel have indications the nerve agent sarin was used by Syrian rebel forces, not by government forces.
That theory was rejected by U.S. officials. 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, Defense Secretary Chuck Hagel said arming the Syrian rebels was a policy consideration.
Until now, U.S. 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 U.S. 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 no-fly zones. However, arming the rebels is the most likely escalation, officials said.
While the Israeli actions have made Kerry's Russia efforts more unpredictable, some in Congress tried to be optimistic.
Maryland Rep. Dutch Ruppersberger, the top Democrat on the House Intelligence Committee, said he hopes Kerry can persuade Russia to use its influence to convince the besieged Syrian leader that he should step down.
"Hopefully the cooperation on the (Boston) Marathon bombing will open the door there," Ruppersberger said.
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 meetings with Israeli Justice Minister Tzipi Livni and Jordanian Foreign Minister Nasser Judeh to discuss Middle East peace prospects.