Syrian troops and Hezbollah allies Friday pressed their assault on the strategic town of Qusayr, as concern grew for residents and hundreds of rebels reportedly broke through army lines to join the battle.
US Secretary of State John Kerry raised questions about Russia's commitment to peace in Syria, warning that delivering S-300 air defence missiles to Damascus would be "not helpful".
In Istanbul, the deeply divided Syrian opposition announced agreement on expanding its membership to include more representatives of fighters and activists on the ground after it was accused of being out of touch.
But despite extending its talks for five days longer than scheduled, the National Coalition postponed electing a new leader and forming an interim rebel government until June.
President Bashar al-Assad, meanwhile, said he was "very confident" of victory.
Assad, whose forces are battling alongside fighters from Lebanon's Shiite movement Hezbollah to recapture Qusayr, said his regime would defeat the revolt which has raged since March 2011 at an estimated cost of more than 94,000 lives.
"There is a world war being waged against Syria and the policy of (anti-Israeli) resistance... (but) we are very confident of victory," he told Hezbollah's Al-Manar television on Thursday.
Syrian state television said troops and Hezbollah fighters captured the northern district of Arjun in Qusayr on Thursday, leaving rebels little chance to escape.
The Syrian opposition said Friday that hundreds of rebel reinforcements, most close to the Muslim Brotherhood, have now reached Qusayr.
"Around 1,000 fighters from across Syria" have penetrated the rebel stronghold near the Lebanese border, the National Coalition's interim leader George Sabra told reporters in Istanbul.
"Hundreds" of rebels have broken through army lines near the village of Shamsinn, northeast of Qusayr, after losing 11 fighters, according to Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The Coalition has appealed for the rescue of 1,000 civilians wounded in Qusayr, which Assad's forces have been trying to retake in an all-out offensive since May 19.
Thousands of people who have fled the besieged town are in dire need of aid, the UN's refugee agency said, as its tally for Syrians who have escaped their war-torn nation topped 1.6 million.
UNHCR spokesman Dan McNorton told reporters that at least 3,500 people -- mostly women and children -- had made it to nearby Hasiya.
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The agency was able "to witness the dire humanitarian situation of these displaced families," he said.
In his interview, Assad appeared to imply that Russia had already started to deliver sophisticated S-300 missile systems under a contract with Syria.
But Russia's Vedomosti and Kommersant newspapers said Moscow may not deliver the missiles this year, and rejected claims they had already arrived.
Russian news agency Interfax reported that Moscow could supply 10 ultra-modern MiG-29 fighter jets to Syria under a possible contract being discussed with Damascus.
The EU on Friday formally waived the arms embargo against rebels, leaving members free from June 1 to decide at their discretion whether to supply carefully vetted weapons to the opposition.
But there was a joint commitment to refrain from supplying weapons "at this stage" for fear of endangering a US-Russian initiative for a peace conference in Geneva, with a review of this set "before August 1".
S-300 missiles In Washington, Kerry warned that Russia's planned delivery of S-300s to Syria is "not helpful" to the joint peace bid.
"In Geneva, we will test who is serious. Are Russians serious about pushing for that? I believe they are. President (Vladimir) Putin said they are, (Foreign Minister) Sergei Lavrov has said it," Kerry said.
"Now, it is not helpful to have the S-300 transferred to the region while you are trying to organise this peace and create peace," he added.
Meanwhile, the UN Security Council added rebel Islamist group Al-Nusra Front to its global sanctions list because of its links to Al-Qaeda, making it subject to an international assets freeze and arms embargo.
On the ground, the Observatory reported that Syrian troops had killed three Westerners, including a US woman and a British man, both Muslims, near the border with Turkey on Wednesday.
The Foreign Office in London confirmed a Briton had been killed, while the family of a US woman believed to have also been killed was anxiously awaiting news.
The Observatory also reported more than 30 prisoners killed in 10 days of clashes between rebels and loyalist forces at the main Aleppo prison, which holds around 4,000 inmates.
In Geneva, doctors from the Union of Syrian Medical Relief Organisations told AFP its members had seen "dozens of cases" of patients suffering from what they believe are chemical weapons attacks, mostly civilians.