ROME - Secretary of State John Kerry said Thursday that the transfer of advanced missile defense systems from Russia to Syria would be a "destabilizing" factor for Israel's security.

Kerry said the United States has expressed concerns about what such defensive systems in Syria would mean for Israel's security. He wouldn't address what the missiles might mean for Syria's civil war.

He spoke to reporters in Rome after the Wall Street Journal reported that Russia was preparing to sell the weapons to President Bashar al-Assad's regime.

Coming just days after Kerry hailed what he described as a U.S.-Russia breakthrough on Syria, the report suggested Moscow may already be angling to further strengthen the Assad regime two years into a war that has killed more than 70,000 people.

"We have previously stated that the missiles," Kerry said, "are potentially destabilizing with respect to the state of Israel."

"We have made it crystal clear that we prefer that Russia would not supply them assistance," Kerry told reporters alongside new Italian Foreign Minister Emma Bonino. "That is on record. That hasn't changed."

White House spokesman Jay Carney, speaking to reporters aboard Air Force One, said, "We have consistently called on Russia to cut off the Assad regime's supply of weapons," including air defense systems that destabilize the region.

"The provision of additional weapons to the regime will not hasten a political solution," Carney said.

Israeli officials said they have asked Russia to cancel the imminent sale to the Assad regime of advanced ground-to-air missile systems.

Such weapons would enhance the Syrian government's defensive ability and make it even harder for the U.S. and other governments to consider even the possibility of trying to enforce a no-fly zone in the country or otherwise intervening militarily.

Russia rarely comments publicly on arms sales or transfers, and there has been no official word on the deal in Moscow.