Assad told Lebanese television channel al-Manar that a Russian contract to supply the government with S-300 air defense missile systems was being implemented but he didn't confirm delivery, the BBC reported Friday.
Israel has said it would attack if missiles were used against it and has carried out airstrikes to stop weapons being sent to Hezbollah in Lebanon.
"Delivery [of the S-300] has not taken place -- I can attest to this -- and I hope it does not. But if, by some misfortune, they arrive in Syria, we will know what to do," Israeli Defense Minister Moshe Ya'alon said.
Israel urged Russia to cancel the arms sale, arguing that the S-300 surface-to-air missile system would enable Assad to more easily transfer chemical weapons to Hezbollah, Israel Today reported Thursday.
Meanwhile, heavy fighting was reported in the Qusair. A Syrian doctor told the BBC more than 600 injured people trapped in rebel-held districts had no access to medical assistance.
"They are waiting three to four days for drinking water and that doesn't include the water they need for everyday use for washing their clothes and for normal day-to-day activities," he said.
Gen. Selim Idriss, military chief of the Free Syrian Army, told the BBC Wednesday more than 7,000 Hezbollah fighters were participating in attacks on Qusair.
U.S. and British officials are investigating reports that an American woman and a British man were killed in Syria while fighting for opposition groups, the BBC said. A report aired by Syrian state TV showing bodies and identity cards apparently belonging to Westerners killed in Idlib province.
Speaking on Lebanon's al-Manar TV, owned by Syrian government ally Hezbollah, Assad warned, "We have informed all the parties who have contacted us that we will respond to any Israeli aggression next time."
"There is clear popular pressure to open a new front of resistance in the Golan," Assad said.
Israel has occupied the Golan Heights since the 1967 war.
Syrian shells hit Israeli positions on the Golan Heights -- although it wasn't clear whether they were directed at rebels in nearby border areas -- and Israel has returned fire.
Excerpts from the al-Manar interview quoted Assad as saying Syria had received a shipment of S-300 surface-to-air missiles from Russia, the BBC said.
However, in the interview, Assad said, "All we have agreed on with Russia will be implemented and some of it has been implemented recently and we and the Russians continue to implement these contracts."
Two Russian newspapers Friday, quoting sources, said it was unclear if any missiles would be delivered this year.
Israeli intelligence sources told to DebkaFile.com -- an open-source military intelligence website in Jerusalem -- Thursday Syria hadn't received the first shipment of the Russian anti-missile batteries.
Ahead of the al-Manar TV interview, an Israeli government official said Israel would "take actions" to ensure that advanced weaponry didn't reach militant groups such as Hezbollah.
Israel has conducted three airstrikes on Syria to stop suspected weapons transfer to Hezbollah.
On Friday, Lebanese Prime Minister Tamman Salam told France's Le Figaro that Hezbollah's involvement in Syria was "not helping matters."
Assad also said during the interview Syria would "in principle" attend a peace conference in June in Geneva if no unacceptable pre-conditions were imposed. The main opposition group outside Syria has said it won't participate in the talks while massacres continue.
Russian, U.S. and U.N. officials are to meet next week in Geneva to prepare for the conference.