THE US and Germany have warned Russia that arming President Bashar al Assad's forces will hurt efforts to bring Syria's warring parties together for peace talks.

US Secretary of State John Kerry, speaking after talks with his German counterpart Guido Westerwelle, said Russian plans to send a sophisticated air defence system to Mr Assad also put Israel's security at risk.

The US and Russia are pressing for a peace conference in Geneva next month between Syria's Government and rebels. Mr Westerwelle called on Mr Assad to "stop the violence and come to the negotiating table" and told Russia sending S-300 missiles to Mr Assad "is totally wrong."

Russia, an ally of Mr Assad, said it would fulfil an order for the long-range surface-to-air missiles from Syria as a deterrent against foreign military intervention.

In a sign Moscow would fulfill other deals, the head of aircraft maker MiG was quoted as saying he was counting on providing Syria with at least 10 MiG-29 fighters.

A Russian arms industry source said the anti-aircraft missiles would probably only arrive in Syria after several months, but more Israeli air strikes or the creation of a no-fly zone would speed up delivery.

The arms industry source said: "Regarding the deliveries of the S-300, they can begin no earlier than the autumn.

"Much will depend on how the situation develops in the region and the position of Western countries."

Syria's opposition cast doubt yesterday on hopes of meaningful progress in peace talks after Mr Assad said only a referendum could decide if he should leave power.

The acting head of the opposition coalition said it would stay out of any such talks as long as Lebanese Hizbollah guerrillas were fighting alongside Assad's forces.

Mr said: "The Syrian coalition will not participate in international conferences and will not support any efforts in light of Hizbollah and Iran's militia invasion of Syria."

However, Mr Sabra's colleagues were more cautious.

A spokesman said the coalition had not made a final decision on whether to go to Geneva.

Other officials said Mr Sabra's view did not necessarily represent that of the wider opposition.

Russia, Mr Assad's main ally, has accused the Syrian opposition of undermining the prospects for negotiations by voting to take part only if a deadline is set for an internationally guaranteed settlement based on Mr Assad's exit.

Meanwhile hundreds of rebels from northern Syria managed to enter the besieged city of Qusair, activists said, to help opposition forces who were battling government troops backed by Hizbollah.

The Syrian Observatory for Human Rights said hundreds of fighters from the Islamist Tawheed brigade, had entered the town.

The two-week battle for Qusair is aimed at securing supply routes near the Syrian-Lebanese frontier.

Seizing Qusair would also allow Assad him to cement control of a belt of territory between the capital, Damascus, and his stronghold on the Mediterranean coast.

Fighters coming to support Qusair's embattled rebel forces were bogged down for days on the outside the town, home to 30,000 people.

Rebels have lost two-thirds of the town to Assad and Hizbollah forces, but say they are still dug in in central Qusair.

They had faced a tight blockade that prevented fighters or supplies from entering and the wounded from leaving.

Rami Abdelrahman, head of the observatory, said it was unclear whether or not the fresh influx of Tawheed fighters was enough to help the opposition regain some lost ground.

He said: "It is too soon to tell if they can make a difference. We will have to watch today and see if they can help the fighters create a turning point."