BEIRUT — Despite recent rebel setbacks in Syria's civil war, the main opposition bloc signaled a tough line Tuesday on attending possible peace talks with President Bashar Assad's regime.

Two senior members of the Syrian National Coalition said the group first wants ironclad guarantees of Assad's departure as part of any transition deal and more weapons for rebel fighters. The group's final position is to be hashed out in a three-day meeting of its General Assembly in Istanbul, Turkey, later this week.

Tuesday's comments highlighted the wide gaps between many in the Syrian opposition and the regime just weeks before the U.S. and Russia hope to bring the sides together at an international conference in Geneva.

Over the weekend, Assad also presented a hard line, challenging the idea of transition talks and saying he won't step down before elections are held. Hours after those comments, his troops launched an offensive against a rebel-held town in western Syria, the latest in a series of military gains by the regime.

"There are many obstacles facing the conference," Lakhdar Brahimi, the U.N.-Arab League envoy to Syria and lead organizer of the gathering, acknowledged Tuesday, after meeting with the Arab League chief in Cairo.

Much about the conference remains up in the air, including the date, the agenda, timetable and list of participants. Brahimi said the conference, initially envisioned for late May, should be held in June at the latest.

The goal is to launch talks between the regime and the opposition on a transitional government in Syria – an idea that was first adopted by the international community in Geneva a year ago but never got off the ground.

Earlier this month, the U.S. and Russia decided to give diplomacy another try, even though they have been backing opposite sides in the 26-month-old conflict that has killed an estimated 70,000 people. The joint effort was quickly overshadowed by disagreements, particularly over Russian shipments of advanced missiles to Assad, deemed ill-timed and unhelpful by the U.S.

The latest signals from Assad and his Russian allies have left the Syrian National Coalition skeptical about the international conference, said Louay Safi, a member of the group's decision-making political office.

"We are serious about having negotiations that would lead to a political solution," Safi said. "But if Assad is not serious, we are not going there (to the conference) for a photo op."

One of the main sticking points is Assad's fate. At Russia's insistence, a compromise at last year's Geneva conference left open the door to Assad being part of a transitional government – a non-starter for the SNC.

"We have been very clear that any transitional period must start with the departure of Assad and the heads of the security services," Khalid Saleh, the spokesman of the SNC, said Tuesday.

He said the Syrian opposition wants guarantees before the start of transition talks that Assad will go. Since the revival of the Geneva plan, the U.S. has remained vague, saying Assad can't be part of a transition, but stopping short of making that a condition for negotiations, as the SNC demands.

Saleh also said the Free Syrian Army, the main Western-backed umbrella group of fighters, must receive "major shipments of weapons" to counter the regime's current military gains. "The FSA must be able to control more areas of Syria before we start thinking about the conference," he said.

The West, particularly the U.S., has been reluctant to arm the rebels, amid concerns such weapons will fall into the hands of Islamic militants with ties to the al-Qaida network.

Britain and France have been breaking out of that consensus in recent weeks, arguing that Assad will only negotiate seriously if the rebels can pressure him militarily.

Arming the rebels should be considered if it becomes clear that Assad is not negotiating in good faith, British Foreign Secretary William Hague said Monday. "We must make it clear that if the regime does not negotiate seriously at the Geneva conference, no option is off the table," he said.

Another sticking point is the list of participants.

The SNC's Safi said the coalition won't attend if many other opposition representatives do as well. The opposition remains fractured among rival groups, though the coalition has been recognized by its Western and Arab sponsors as the legitimate representative of the Syrian people.

Haitham Manna, a leader of one of the rival groups, the National Coordination Body, said the coalition should not attend peace talks alone. Unlike the still largely exile-based SNC, Manna's alliance of 16 groups has roots in Syria and is more open to compromise with members of the regime, though not with Assad.

"The military way is a dead end, there can be no winners," Manna said. "And if there is a winner, he will leave behind enough hatred to turn every loser into a suicide bomber."

A senior U.S. official said Tuesday that no opposition group has definitely decided to take part. He noted that as part of the SNC's meeting in Istanbul this week, the group will also choose a new leader, and that the U.S. hopes to persuade the new leadership to attend the conference.

The U.S. official spoke with reporters traveling with Kerry in Muscat, Oman, He spoke on condition of anonymity because he was not authorized to talk publicly about Syria diplomacy ahead of Kerry's trip.

The U.S., Russia and several other nations will also participate in the conference, if it takes place. Obama administration officials have refused to rule out the participation of Assad's biggest military backer, Iran.

Kerry, meanwhile, will meet with 10 of America's closest Arab and European allies in Jordan on Wednesday.

One of the aims of the meeting is to find a way to change Assad's calculation, only fortified by his recent military successes, that he can win militarily, the U.S. official said.

He declined to say what the U.S. might include in its next package of nonlethal aid to the Syrian rebels, which still has to be notified to Congress. He also didn't signal any imminent move by the Obama administration to provide lethal support.

In Syria, regime troops were trying for a third day Tuesday to wrest control of the western town of Qusair from the rebels. The town lies along a strategic land corridor linking the capital Damascus with the Mediterranean coast, the heartland of Assad's Alawite sect.

UNICEF said it was "extremely concerned" about the safety of civilians in Qusair. The U.N. child protection agency said up to 20,000 civilians, many of them women and children, could be trapped there by the fighting.

Also Tuesday, Israeli and Syrian troops exchanged fire across their tense cease-fire line in the Golan Heights, prompting an Israeli threat that Syria's leader will "bear the consequences" of further escalation and raising new concerns that the civil war could explode into a region-wide conflict.

The incident marked the first time the Syrian army has acknowledged firing intentionally at Israeli troops since the civil war began. Assad's regime appears to be trying to project toughness in response to recent Israeli airstrikes near Damascus.

In Geneva, U.N. officials said the number of Syrian refugees arriving in Jordan has suddenly fallen from an average of 2,500 a day to fewer than 20.

Millions of people have been displaced in the civil war, and Jordan has taken in hundreds of thousands of them. U.N. officials said they are unsure what has led to the drop in the flow of refugees to Jordan this week. They said they lack staff on the Syrian side of the border and cannot observe the situation there.

___

Klapper reported from Muscat, Oman. Associated Press writers Zeina Karam in Beirut, Aya Batrawy in Cairo, John Heilprin in Geneva and Josef Federman in Jerusalem contributed reporting.

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