A meeting of Syria's fractured opposition headed into an unscheduled fourth day Sunday, while on the ground rebels kept up their resistance to a Hezbollah-backed government assault on a strategic central town.
Hezbollah chief Hassan Nasrallah on Saturday pledged "victory" in Syria over the rebels and stressed it was in his militant anti-Israeli group's own interest to defend the Assad regime.
"I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one" in Syria, he said in a speech for the 13th anniversary of Israel's withdrawal from Lebanon.
As the fighting continues, the main opposition National Coalition has met in Istanbul for three days trying to overcome deep divisions over Russian and US proposals to convene a conference to which representatives of President Bashar al-Assad would be invited without any formal precondition for him to step down.
The opposition's longstanding position is that, after more than two years of devastating conflict which has killed more than 94,000 people, it will not negotiate until Assad agrees to leave.
Some within the Coalition said it should negotiate if talks lead to Assad's departure, while others have expressed reservations.
Delegates said efforts to reach an agreed position on the proposed conference were being delayed by pressure from some of the opposition's Gulf Arab backers for an overhaul of its membership that was being resisted by other governments.
A bid by oil powerhouse Saudi Arabia to weaken the Muslim Brotherhood and to hold sway over the Coalition has overshadowed the debate, dissidents said.
"You have Saudi Arabia and the United Arab Emirates pushing to include up to 30 new members in the National Coalition," a Coalition member said on condition of anonymity.
"Their goal is to downsize the Muslim Brotherhood's influence over the group," he added.
The National Coalition is currently dominated by the Syrian National Council, in which the main political bloc is the Muslim Brotherhood.
Delegates said the meeting, scheduled to end Saturday, was set to continue in a bid to reach agreement.
"I hope that we will have some good news tomorrow," Coalition spokesman Khaled al-Saleh told reporters late Saturday.
Nasrallah's victory pledge followed rare criticism of Hezbollah on Friday by Lebanese President Michel Sleiman, who cautioned it against its intervention in Syria.
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Last week government troops launched an assault on Qusayr and the intervention of hundreds of fighters of Shiite militant group Hezbollah from neighbouring Lebanon has given the regime the upper hand in the battle.
Loyalists overran a disused military airport on Saturday just north of the besieged town, where the rebels had set up base, a military source said.
But six days after the assault began, fierce resistance continued from the rebels, for whom Qusayr provides an important supply line for arms and volunteers from nearby Lebanon.
"The fighting and shelling, which took place on Saturday on the main roads inside and outside of Qusayr, are the most intense since the beginning of the offensive," said Syrian Observatory for Human Rights director Rami Abdul Rahman.
The Observatory said at least 27 rebels and three civilians were killed, including a child.
Qusayr is a key prize for Assad because of its strategic location between Damascus and the Mediterranean coast, the Alawite heartland of the embattled president's regime.
Of concern to world powers is that the offensive in Qusayr has sparked renewed clashes between Assad supporters and opponents inside Lebanon.
Fighting between them in the Lebanese port of Tripoli has killed 30 people since May 19, a security source said.
Israel accused Syrian forces of trying to provoke conflict as the two sides rowed over responsibility for a border clash.
Tensions around Golan Heights ceasefire zone between the two rivals have escalated since Syrian forces fired across the UN-patrolled ceasefire line and hit an Israeli military vehicle on Tuesday.
"This week's events are part of a disturbing pattern of events intended to spark provocation with Israel," said Israel's UN ambassador Ron Prosor in a letter to the UN Security Council on Saturday.
Iranian Defence Minister Ahmad Vahidi on Saturday denied accusations by US officials that his country had troops on the ground alongside Hezbollah. "The Islamic Republic of Iran has never sent military forces to Syria and will never do so," he said.
Meanwhile Syria's eastern neighbour Iraq launched a major security operation in the border region deploying 20,000 troops to clear suspected rebel rear bases and secure a key highway, senior officers said.
The opposition Coalition, wrong-footed by Moscow's announcement that regime representatives had agreed to attend next month's planned peace conference, called on Damascus to give concrete evidence of its readiness for a transition of power.
The United States and Russia, which support opposite sides in Syria's conflict, are pushing for the conference. On Monday US Secretary of State John Kerry will meet Russian counterpart Sergei Lavrov in Paris to step up their efforts to organise it.