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The Syrian opposition is demanding access to arms before planned peace talks next month, amid a growing consensus that it may take a shift in the balance of power on the battlefield before any meaningful negotiations can take place.

Khalid Saleh, a spokesman for the Syrian Opposition Coalition, said Thursday that his group is pushing for “serious arming” of the rebels ahead of the U.S.- and ­Russian-backed negotiations. His comments come as European nations consider linking the lifting of an E.U. arms embargo to the talks.

Syrian President Bashar al-
Assad’s forces have made gains on the ground in recent weeks, closing in on the strategic western city of Qusair with the help of fighters from Lebanon’s Shiite Hezbollah movement. On Thursday, they repelled a rebel attack on a prison in the northern city of Aleppo.

Analysts and diplomats say that those gains may dim the chance of the regime taking part in substantive talks and that the opposition needs to be strengthened. Meanwhile, the opposition has jumped at the opportunity to leverage its long-standing demand for arms.

“We’re not going to sit at the table while Assad continues to kill, supported by Russia and Hezbollah,” Saleh said. “What we are asking for is arming the Free Syrian Army or Supreme Military Council — before the talks.”

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

Although President Obama said Thursday that his administration will continue to ratchet up pressure on Assad’s government, it remains reluctant to provide direct military aid. However, Britain and France are pushing the European Union not to renew an arms embargo on Syria that expires May 30, hoping to use the prospect of weapons shipments as an inducement to the opposition to join talks while simultaneously increasing pressure on Assad.

“We need a stronger, more capable opposition that can make Assad feel vulnerable,” said one European diplomat who spoke on the condition of anonymity to discuss sensitive developments. “That is why amending the arms embargo now is so important.”

Assad “will know that if he does not engage seriously in negotiations, we have the option of sending lethal support to the opposition to force his hand,” the diplomat added.

The push comes despite concerns about the makeup of the opposition and the prevalence of jihadist groups, compounded by the emergence this week of a video that purports to show a rebel commander biting into the organ of a dead soldier.

The peace talks, scheduled for mid-June, are beset with uncertainties, including who will represent both sides. Robert Ford, the U.S. ambassador to Syria, traveled to southern Turkey last week to meet with opposition leaders about the talks.

The Syrian government has given Russia a list of officials it has selected to represent it at the negotiations, but both sides have said that they need more details before committing. Saleh said there remains a “tremendous amount of ambiguity.”

“Everything’s up in the air,” said Salman Shaikh, director of the Brookings Doha Center. “The opposition will try to get any leverage it can, and right now we are in a situation when the Assad regime really does believe it’s winning, so there’s very little incentive for them to give anything.”