Syria's Assad warns Israel on air strikes
Syria's President Bashar al-Assad has warned Israel that it will respond in kind to any future air strikes.
Mr Assad told a Lebanese TV channel a Russian contract to supply it with air defence missiles was being implemented - but did not confirm any deliveries.
Israel has warned it will attack if the system is used against it. It has carried out strikes to stop weapons being sent to Hezbollah in Lebanon.
Meanwhile heavy fighting continues in the strategic Syrian town of Qusair.
A Syrian doctor has described the horrors of living there, telling the BBC that more than 600 injured people were trapped in rebel-held districts with 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.
There were women and children "dying in the battle for more control" of the town, which lies 30km (18 miles) south-west of Homs, he said.
He said he had seen the bodies of "many" fighters from Hezbollah, the Lebanese Shia movement.
Gen Selim Idriss, the military chief of the main umbrella group of Syrian rebels, the Free Syrian Army, told the BBC on Wednesday that more than 7,000 Hezbollah fighters were taking part in attacks on Qusair.
Meanwhile, US and UK officials are looking into unconfirmed reports that an American woman and a British man have been killed in Syria.
This follows a report aired by Syrian state TV showing the bodies and identity cards apparently of Westerners killed by government troops while fighting for the rebels in north-west Idlib province.
The woman's family later reported her death, naming her as Nicole Mansfield, a 33-year-old Muslim convert from the town of Flint, Michigan.
The UK Foreign Office said on Friday it was aware of the reports about the British citizen and was seeking information.
Speaking to Lebanon's al-Manar TV, which is owned by Hezbollah, a close ally of the Syrian government, Mr Assad warned: "We have informed all the parties who have contacted us that we will respond to any Israeli aggression next time."
He added: "There is clear popular pressure to open a new front of resistance in the Golan."
Israel has occupied the Golan Heights since the 1967 war. It annexed the territory in 1981, in a move that has not been recognised by the international community.
Syrian shells have hit Israeli positions on the Golan Heights, though it is unclear whether they were aimed at rebels in border areas, and Israel has returned fire.
Syria and Israel have been in a state of war since 1948 but the border had been relatively calm in recent years.
Excerpts released from the al-Manar interview ahead of broadcast quoted Mr Assad as saying Syria had already received a first shipment of S-300 missiles from Russia.
But in the interview itself, he said only: "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."
The S-300 is a highly capable surface-to-air missile system that, as well as targeting aircraft, also has the capacity to engage ballistic missiles.
Two Russian newspapers on Friday quoted sources as saying that it was unclear if any of the missile system would be delivered this year.
Russia's MiG company says it is also discussing the supply of more MiG-29 M/M2 fighter planes to Syria.
General director Sergei Korotkov said the number would be "more than 10".
Ahead of the Manar TV interview, Israeli government minister Silvan Shalom said Israel would "take actions" to ensure that advanced weapons did not reach groups such as Hezbollah, but there was no need to "provoke an escalation".
He told public radio: "Syria has had strategic weapons for years, but the problem arises when these arms fall into other hands and could be used against us. In that case, we would have to act."
Israel has already carried out three air strikes on Syria to stop the transfer of advanced weapons to Hezbollah in Lebanon.
On Friday, Lebanese PM Tamman Salam told France's Le Figaro that Hezbollah's involvement in Syria was "not helping matters" and that Lebanon should stay out of the conflict.
Mr Assad also said Syria would "in principle" attend a peace conference next month in Geneva backed by the US and Russia, if there were not unacceptable preconditions.
The main opposition group outside Syria has said it will not join the talks while massacres continue.
Its interim leader, George Sabra, said talk of diplomatic conferences was farcical while Syrian government forces backed by Hezbollah were carrying out heinous crimes.
The opposition has been meeting in Istanbul to elect new leaders and devise a strategy.
But the BBC's Jim Muir, in Beirut, says it appears in disarray after eight days of bitter internal wrangling and that a lot needs to be done for it to be in any kind of shape to attend any conference in a coherent manner.
For his part, Mr Assad said it would not be surprising if the conference failed, and if it did, it would not make much difference on the ground, because what he called the "terrorism" of the rebels would continue.
Russian, US and UN officials will meet next week in Geneva to prepare for the proposed conference.
How the Russian S-300PMU-1 missile defence system works
- The long-range surveillance radar tracks objects over a range of 300km (185 miles) and relays information to the command vehicle, which assesses potential targets.
- A target is identified and the command vehicle orders the engagement radar to launch missiles.
- Launch data is sent to the best placed of the battalion's six launch vehicles and it releases two surface-to-air missiles.
- The engagement radar helps guide the missiles towards the target. It can guide up to 12 missiles simultaneously, engaging up to six targets at once.