ROME — Secretary of State John Kerry cautioned Russia on Thursday against selling advanced surface-to-air missiles to Syrian government forces, as US 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 denounce the reported agreement between Russia and Syrian President Bashar Assad directly, but his warning to Russia 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 Kerry won in Moscow this week to press jointly for peace talks between Assad and his US-backed opponents.
Moscow has previously sparred with Western governments over sales of helicopter and tank parts to the embattled regime. But the S-300 missiles, if delivered, would represent a significant leap in Syria’s ability to defend itself from Israeli airstrikes as well as any future effort to impose a no-fly zone in support of Syria’s rebels.
Considered one of the most potent air-defense systems, the S-300 system 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 insisted on anonymity because details of the alleged offer remain classified, speculated that Moscow could be seeking leverage in advance of talks on a possible political settlement.
Russia was heavily criticized in 2007 when it signed a deal to sell S-300 batteries to Iran for $800 million. Eventually Russian officials terminated the contract, citing new UN resolutions banning the export of advance missile systems to Tehran.
Meanwhile, Hezbollah leader Hasan Nasrallah vowed Thursday that Syria would provide the Shi’ite Lebanese movement with even more powerful weapons to supplement those destroyed by Israel in a series of airstrikes against Damascus over the weekend, but he refrained from threatening retaliation.
In the first official response by Hezbollah to the strikes, Nasrallah said Syria would provide his movement with ‘‘game-changing’’ weapons that would ‘‘break the balance’’ of power in the region.
‘‘Syria will give more weapons, better quality weapons, to the resistance than the resistance has ever had before,’’ he said in a speech broadcast by Hezbollah’s al-Manar TV station. ‘‘This is the strategic response of Syria.’’
Nasrallah also pledged to provide moral and material support to an unspecified Syrian resistance movement that would attempt to recover the Golan Heights, occupied by Israel since 1967.
However, the speech appeared to rule out any immediate Hezbollah response to the strikes, in which Israeli warplanes demolished targets said to include shipments of Fateh-110 missiles destined for the Shi’ite movement. Hezbollah plays a commanding role in Lebanon’s government and also sustains a private army for the chief purpose of confronting Israel.