Syrian troops and their Hezbollah allies tightened the noose on the last rebel-held parts of the strategic central town of Qusayr on Friday, after President Bashar al-Assad said he was "very confident" of victory.
Assad also threatened Israel with retaliation for any attack in the interview aired late on Thursday. There was no immediate comment from Israeli officials.
In Istanbul, the deeply divided opposition announced agreement in the early hours on expanding its membership to include more representatives of fighters and activists on the ground following accusations it was out of touch.
But despite extending its talks for five days longer than scheduled, the National Coalition put off until June the election of a new leader and the formation of the interim rebel government demanded by the Arab League as a condition for giving it Syria's vacant seat in the bloc.
Assad, whose forces are battling alongside Hezbollah fighters to recapture Qusayr, said he was "very confident" of victory.
"There is a world war being waged against Syria and the policy of (anti-Israeli) resistance... (but) we are very confident of victory," he said in the interview with the Lebanese militant group's Al-Manar television.
Syrian state television said troops and Hezbollah fighters had captured the Arjun district in the north of Qusayr on Thursday, leaving rebels holed up in the town little chance to escape.
The town is a strategic prize as it controls supply routes vital to both sides.
Lebanese Prime Minister Tamman Salam said his country should stay out of the Syrian conflict, in an interview published in Friday's edition of French daily Le Figaro.
"We must at all price preserve national unity," he added.
"And obviously, Hezbollah's military involvement is not helping matters."
Assad threatened Israel with renewed fighting on the Golan Heights where a UN-patrolled armistice line has separated the two armies since Israel captured much of the strategic plateau in the 1967 Middle East war.
"There is clear popular pressure to open a new front of resistance in the Golan," he said.
"There are several factors, including repeated Israeli aggression," he added, referring to reported Israeli air strikes on Syria.
"We have informed all the parties who have contacted us that we will respond to any Israeli aggression next time."
There was no immediate comment on Assad's remarks from Israel, which has said it does not want to provoke a military "escalation" with Syria, but will not allow it to transfer strategic arms to groups like Hezbollah.
Assad appeared to imply in the interview that Russia had already delivered some of the promised S-300 missiles that have sparked particular concern in Israel.
"All the agreements with Russia will be honoured and some already have been recently," he said.
But Russia's Vedomosti and Kommersant newspapers said Moscow may not deliver the missile systems to Damascus this year and rejected claims the weapons had already arrived.
Washington warned that any such weapon deliveries from Moscow would only prolong the conflict, which activists say has killed more than 94,000 people since March 2011.
Moscow said earlier this week that it would honour the contract to deliver the S-300s to Syria, saying the missiles were a "stabilising factor" which could act as a deterrent against foreign intervention.
A monitoring group meanwhile 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.
"They were shot dead during an ambush in the Idlib region and the army found them with maps of military positions," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
They had apparently been taking photos of military positions on the road between Harim, near the border with Turkey, and the town of Idlib further south, he added.
The United Nations said a preparatory meeting for a proposed international conference on the Syrian conflict would take place in Geneva next Wednesday, attended by Russian, US and UN officials.
Russia says Assad's regime has agreed to attend the proposed conference but the opposition National Coalition has said it will not take part "so long as the militias of Iran and Hezbollah keep up their invasion".
The Coalition has appealed for the rescue of 1,000 civilians wounded in Qusayr, which Assad's forces have been trying to seize back in an all-out offensive since May 19.
As the Coalition wrapped up its talks in Istanbul early Friday, it announced it would admit 43 new members, including 15 representatives of the rebel Free Syrian Army's high command and 14 of revolutionary movements inside Syria.
But it failed to elect a new leader to replace Ahmad Moaz al-Khatib, who resigned in March, and also failed to form a long-promised interim government to administer rebel-held areas.