Syrian soldiers have killed three Westerners, including an American woman and a British man, both Muslims, in northwest Idlib province near the Turkish border, a monitoring group says.
"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.
The three 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 when government troops ambushed them, he added.
The nationality of the third Westerner was not clear, said Rahman.
A US State Department official in Washington said: "We are aware of the case."
Czech diplomats in Syria were helping them in efforts to get more information.
The family of a 33-year-old Michigan woman, Nicole Lynn Mansfield of Flint, says the FBI had confirmed that she was killed in fighting in Syria.
A relative said her niece was a convert to Islam who married an Arab immigrant several years ago but later divorced him.
In Syria, President Bashar al-Assad dared his foes to take him on, promising to respond to any Israeli strike on his country, as he appeared to indicate that Russian missiles were already on their way.
In an interview aired on both Hizbollah's television channel in Lebanon and Syria's own state media, Assad said Russia had fulfilled some of its weapons contracts recently, but was vague on whether this included advanced S-300 air defence systems.
Assad surprisingly appeared to acknowledge that Russia's decision to go ahead with the delivery of the defence system, announced by Moscow this week, was not a response to the EU's decision to lift the arms embargo.
"We have been negotiating with them about different types of weapons for years and Russia is committed to Syria to implement these contracts. 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."
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His comments differed from excerpts from the TV station released in advance, which suggested Syria had already received a first shipment of such missiles.
Russia said on Wednesday that the delivery was to discourage Western intervention, in particular to stop moves towards imposing a no-fly zone against Assad's forces, as happened in Libya. The weapons system is geared to counter attacks by Israeli jets, such as those which have attacked sites in Syria suspected of supplying heavy arms to Hizbollah, Israel's foe in Lebanon.
Moshe Yaalon, Israel's Defence Minister, said he did not think the weapons systems had been sent, but he added: "If God forbid they do reach Syria, we will know what to do."
Asked whether Syria would respond to such strikes, Assad said: "We have told countries that we will respond to any strike with a similar strike", saying the manner of the response "depends on the circumstance and timing" of the attack.
The S-300, seen as a game changer in any conflict with Assad's potential enemies, would be a major obstacle to a no-fly zone and shows Russia's determination to defend Assad at all costs.
All sides are increasing their rhetoric in advance of a peace conference in Geneva next month. Assad has indicated he will be represented, but there remain sticking points.
Russia insists Iran attends, something opposed by European states. The opposition Syrian National Coalition has said it will not attend as long as "Iran and Hizbollah kept up their invasion" - a reference to the battle for Qusair in the west of the country, in which the Lebanese Shia militia have taken the lead role.