Syrian President Bashar al-Assad threatened Israel with renewed fighting in the Golan Heights and said Russia was committed to supplying him with advanced missiles, in an interview broadcast on Thursday.
Washington warned that any such weapon deliveries from Moscow would only prolong the conflict between government forces and Assad's foes in Syria, where activists say more than 94,000 people have been killed since March 2011.
"There is clear popular pressure to open a new front of resistance in the Golan," Assad told Al-Manar television of his close ally, the Lebanon-based Shiite movement Hizbollah fighting alongside his forces.
"There are several factors, including repeated Israeli aggression," he said, 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 seized the Golan from Syria in the 1967 Six-Day War. The armistice line has remained calm ever since, despite some spillover from the Syrian conflict.
Assad, whose forces are battling alongside Hizbollah fighters to recapture the key town of Qusayr near the border with Lebanon, 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.
Earlier, Syrian state television said the Arjun district in northern Qusayr, one of the few remaining rebel strongpoints, had been taken, leaving rebels there little chance to escape.
Assad, who belongs to the Alawite offshoot of Shiite Islam, appeared to imply in the interview that Russia had already delivered some of the promised ground-to-air S-300 missile systems.
"All the agreements with Russia will be honoured and some already have been recently," he said.
Moscow, the Assad regime's most powerful ally, has yet to confirm if it has already sent S-300s to Syria, but it announced this week that it intends to honour its contract.
US National Security Council spokeswoman Caitlin Hayden declined to comment on Assad's suggestion that S-300s had been delivered.
But she said: "Our concerns about Russia's continued support for the Syrian regime through the provision of arms and access to Russian banks are well known."
"Providing additional weapons to Assad -- including air defence systems -- will only prolong the violence in Syria and incite regional destabilisation," she added.
British foreign secretary William Hague called the stalemate "the most pressing crisis in world affairs today".
Israeli Energy and Water Minister Silvan Shalom indicated that the Jewish state would only act to prevent the missiles being used against it.
Israel has already launched several air raids inside Syria this year, reportedly targeting convoys transporting weapons to its arch-foe Hezbollah.
Russia has defended its arms shipments to Syria. Deputy Foreign Minister Sergei Ryabkov has said the missiles are a "stabilising factor" that could deter 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 meanwhile 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.
Earlier, Syria's opposition National Coalition, meeting in Istanbul, said it would not take part in the peace initiative dubbed Geneva 2 "so long as the militias of Iran and Hezbollah keep up their invasion".
They wrapped up eight days of often troubled meetings in Istanbul early Friday announcing they had expanded to include 43 new members.
They included 15 representatives from the rebel Free Syrian Army's high command and 14 from revolutionary movements inside Syria, said acting chief George Sabra.
The Coalition has appealed for the rescue of a thousand citizens wounded in Qusayr, which Assad's forces have been trying to seize back in an all-out offensive since May 19.
It has also insisted that any negotiations with the regime must lead to Assad's resignation, a position Russian Foreign Minister Sergei Lavrov criticised on Thursday as unrealistic.
"We are under the impression that the National Coalition and its regional sponsors are doing everything so as not to allow the start of the political process and achieve military intervention in Syria through any means possible," said Lavrov.
"These demands are impossible to fulfil," he added. "The only thing that unites them is a demand of Bashar al-Assad's immediate departure."