Secretary of State John F. Kerry on Thursday cautioned Russia against selling advanced ­surface-to-air missiles to Syrian government forces, as U.S. officials warned that the delivery of the arms would threaten Israel and undercut efforts to reach a political agreement.

“We’ve made it crystal clear that we prefer that Russia would not supply them assistance,” Kerry said during a news conference with Italy’s new top diplomat. “That is on record. That has not changed.”

Kerry declined to directly denounce the reported agreement between Russia and Syrian President Bashar al-Assad, but his warning to Moscow was unmistakable.

The United States has long said that the proliferation of surface-to-air missiles “is potentially destabilizing with respect to the state of Israel,” Kerry said. “We have made it very clear historically that that is a concern of the United States.”

Russia has long supplied Assad’s forces, but the potential sale of antiaircraft weapons reported Thursday by the Wall Street Journal threatens to undermine the agreement that Kerry won in Moscow this week to press jointly for peace talks between Assad and his U.S.-backed opponents.

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Moscow has previously sparred with Western governments over sales of helicopter and tank parts to the embattled Assad regime. But the S-300 missiles, if delivered, would represent a huge leap in Syria’s ability to defend against Israeli airstrikes and any effort to impose a no-fly zone in support of anti-Assad rebels.

The S-300 system, considered one of the most potent air-defense systems in use, can track as many as 100 incoming aircraft or missiles at once, and engage up to a dozen, at long range.

“It would be a game-changer,” a senior Western diplomat said of the reported decision to offer the missiles to Assad. The diplomat, who spoke on the condition of anonymity because details of the reported offer remain classified, speculated that Moscow could be seeking leverage ahead of talks on a possible political settlement to the Syrian crisis.

Russia was heavily criticized in 2007 when it signed a deal to sell S-300 batteries to Iran for $800 million. Russian officials eventually terminated the contract, citing new U.N. resolutions banning the export of advanced missile systems to Tehran.

“After discussions with us, they did decide not to provide the missiles to the Iranians,” recalled Dennis Ross, who was a senior Middle East adviser to the Obama administration in 2010, when Russia halted the missile sale to Iran. “If they proceed now, it hardly signals that they are prepared to walk away from Assad.”

Kerry has declined to draw a comparison between the military aid provided by Moscow to the Syrian government and what has, until now, been the U.S. choice to send only humanitarian and nonmilitary assistance to the rebel forces. President Obama is considering whether to reverse that decision and provide arms to the rebels, as some Arab states are doing.

Kerry met earlier Thursday with the foreign minister of Jordan, which is staggering under the weight of more than 500,000 Syrian refugees. Kerry said that about $43 million of a new $100 million U.S. aid package pledged for Syrian refugees will go to Jordan.

“Jordan feels the impact of what is happening more than any other country,” he said.

Jordanian Foreign Minister Nasser Judeh said the refugees account for 10 percent of Jordan’s population, and the figure is expected to at least double this year.

Judeh was headed next to Moscow to discuss a potential Syrian transitional government.

“It has to be a transitional period that results in a political solution that includes all the segments of Syrian society, no exclusion whatsoever,” Judeh said, while preserving “Syria’s territorial integrity and unity.”

Jordan is expected to play a key role in marshaling Arab support for Middle East talks. Jordan was the second Arab state to make peace with Israel, after Egypt.

“There have been many failed attempts, false starts, and there were attempts that resulted in limited success, perhaps, and we should build on all that,” Judeh said in Rome, where he and Kerry arranged their second meeting in as many weeks. Kerry met Wednesday with Israeli Justice Minister Tzipi Livni, who is her government’s chief peace negotiator.

“This is why it’s important to look at the history and share our thoughts and our ideas and our approaches with each other, so that we can try and bring the parties back to the negotiating table, perhaps in a different way and more effective way this time,” Judeh said.

The Obama administration has set no deadline for Kerry’s effort to draw Israel and the Palestinians to the table and has revealed few details of what he is urging each side to consider once talks begin.

Kerry has said that he wants to move quickly. Events in the tumultuous Middle East, or spoilers on either side, have doomed past negotiations.

“Each day that goes by in the Middle East always brings the ability for someone, somehow, to create events that always threaten the ability of the process to continue smoothly,” Kerry said.

Israel’s detention of an important Muslim leader Wednesday for questioning sparked anger among Palestinians, who accused Israel of human rights violations.

Judeh called on both sides to avoid provocations.

Warrick reported from Washington. Ernesto Londoño in Washington contributed to this report.