Syrian President Bashar al-Assad vowed Thursday to press ahead with plans to receive long-range air defense missiles from Russia, while the main Syrian opposition group said it would not participate in peace talks in Geneva, further dampening prospects for a U.S.-backed effort to end the two-year-old conflict.

The Syrian Opposition Coalition, meeting in Istanbul, said it would not attend the proposed talks while militants from Iran and the Lebanese Shiite movement Hezbollah continue to back Assad’s forces. The announcement is a setback for the United States, which has been leading efforts to get the fractured Syrian opposition to the negotiating table. However, the coalition bowed to Western pressure to broaden its ranks, expanding the 71-seat assembly by adding 43 new members, including 15 representatives of the Free Syrian Army.

Russian, U.S. and U.N. officials are scheduled to meet Wednesday for a last-ditch effort to bring the warring sides in Syria together for talks. White House spokesman Josh Earnest said Thursday that the United States would continue to work “aggressively” to organize the Geneva talks.

Assad said Thursday that his government was willing to attend the Geneva talks but that any decision taken there would be implemented only after a vote by the Syrian people.

But there is little incentive for the Syrian leader to engage meaningfully in negotiations. His regime has made slow but meaningful gains in recent weeks, and his forces are advancing in the town of Qusair, backed by Hezbollah fighters.

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“We are confident and sure about victory, and I confirm that Syria will stay as it was,” Assad said in an interview with al-Manar television, a mouthpiece for Hezbollah, that was broadcast Thursday.

A Lebanese pro-Hezbollah newspaper, al-Akhbar, had reported Thursday that Assad would confirm the delivery of the first shipment of Russian S-300 air defense missiles in the prerecorded interview, though Israeli defense experts had raised doubts about the arrival of the missiles.

Israeli officials have indicated that they would act to ensure that the missile system, which has a range of about 130 miles — the distance from Damascus to Tel Aviv — does not become operational.

But Assad was vague during the televised interview about whether the first shipment of the missiles had been delivered. He said he was “committed” to fulfilling existing contracts and, when asked specifically where the shipment was, said, “Everything we agreed upon with Russia will be done; and a part [of those contracts] has already been completed lately.”

Earnest said he could not confirm whether a transfer of weapons had taken place, adding: “We have long made clear that we are concerned about the Russian support for the Assad regime.”

Recent efforts to funnel more weapons to the battlefield have caused both sides to harden their positions — each citing the moves as evidence that the other is not serious about meaningful negotiation.

Despite Russia’s work with the United States to get talks started, and calls from the United States, Israel and France to halt the sale, Moscow pledged to go ahead with plans to send Assad, a longtime ally in the region, one of the most advanced surface-to-air missile systems in the world. At the same time, the European Union’s decision Monday to let a weapons embargo on Syria lapse opened the door to the possibility of Britain and France arming the rebels.

Israel is said to have carried out several airstrikes in recent months on Syrian targets, including a convoy that U.S. officials said was suspected to be transferring weapons to Hezbollah. The potential delivery of S-300 missiles to Syria threatens Israel’s ability to carry out such operations and has raised fears among Israelis that the technology could be passed to Hezbollah.

Michael Herzog, a retired brigadier general in the Israel Defense Forces, said the Russians would probably ship the S-300 system piecemeal. “It would take many months for everything to arrive, and then they would have to assemble it,” he said.

Even after its delivery, it would take the Syrians at least three to six months to learn how to operate the system, said Yiftah Shapir, director of the military balance project at the Institute for National Security Studies in Tel Aviv, who has studied the S-300 extensively.

“It is highly doubtful that the Syrian army, in its current situation, is able to invest the man­power and resources” to learn how to use the S-300, he wrote recently.

“Maybe nothing has arrived, and maybe something has arrived,” Herzog said. “But the important thing for Israel is that the Russians say they are sending it. This is a strategically significant weapon that changes the equation in very dangerous ways. It cannot be ignored.”

Israeli Prime Minister Benjamin Netanyahu met with Russian President Vladimir Putin in the Black Sea resort of Sochi this month to dissuade him from selling advanced antiaircraft missile batteries to Syria.

Israeli national security adviser Yaakov Amidror said at a meeting of 27 European Union ambassadors this week that Israel would not try to stop the delivery of the S-300s but would attempt to stop the state-of-the-art system from being made operational, according to an account in the Israeli newspaper Haaretz.

The S-300 battery is similar to the Patriot system used by the U.S. military and is capable of tracking and striking multiple targets simultaneously. Depending on the model, it can fire highly accurate missiles with ranges of more than 100 miles.

Booth reported from Jerusalem. Ahmed Ramadan in Beirut contributed to this report.