Syria and neighbouring Lebanon were on alert Thursday, said security sources, a day after Israeli jets had reportedly hit a Syrian military research facility.
"Our army units are on full alert after aggression on our sovereignty by the Zionist planes," a Syrian army officer based near the Lebanese border told dpa.
The Syrian army said Wednesday Israeli jets had targeted a military research centre north-west of the capital Damascus, but denied reports that the raid had hit a convoy of weapons near the Lebanese border.
Two people were killed and five injured in the strike, according to Syria. There was no comment from Israel on the incident.
The Lebanese army Thursday intensified patrols mainly in areas bordering Syria.
Trucks with anti-aircraft machine guns were positioned in some areas close to the border, a Lebanese military source said.
The source, speaking on condition of anonymity because of the sensitivity of the issue, said "heavy Israeli air activity" had been registered in the area overnight.
Israel reportedly fears that sophisticated anti-tank and anti-aircraft weapons, supplied to conflict-hit Syria by Russia, could end up with the Lebanese Hezbollah movement, a staunch ally of Damascus.
A Lebanese security source in eastern Lebanon told dpa that the strike was very close to areas also controlled by Hezbollah on the Lebanese side of the border.
Lebanon has denied reports that the strike had taken place inside its territories.
Hezbollah, believed to be financed and trained by Iran, has been accused by the Syrian opposition rebels of fighting on the side of troops loyal to President Bashar al-Assad.
A Western diplomat in Beirut, who asked not to be named, said the Israeli strike has targeted a truck convoy believed to be carrying anti-aircraft weapons for Hezbollah.
He added that some of the weapons which were believed to be in the convoy were sophisticated Russian-made surface-to-air missiles.
Israel believes that Syria received from Russia a battery of SA-17s after Israeli planes had destroyed an unfinished Syrian nuclear reactor in 2007.
Wednesday's strike drew condemnation, mainly from Syria's allies Russia and Hezbollah.
"If the information is confirmed, this would be a grave violation of the UN Charter," the Russian Foreign Ministry said.
Hezbollah said in a statement: "This aggression reveals the conspiracy against Syria that has been going on for the past two years. It's an attempt to demolish Syria and its army and end its role in the axis of resistance."
Syria, Hezbollah and Iran are sworn foes of Israel, making up what they frequently call the "axis of resistance against the Zionist enemy."
The Arab League also criticized Israel, saying the strike was a “flagrant aggression and a glaring violation" of Syria's sovereignty.
European Union foreign ministers meeting in Brussels reacted cautiously to reports of the strike.
"I don't have any additional information," British Foreign Secretary William Hague told reporters.
"Of course, there are great dangers from the Syrian conflict," he added, referring to the nearly two-year civil war.
German Foreign Minister Guido Westerwelle said: "As long as we don't have any own knowledge, as long as we don't have any assured knowledge about what is being reported there, I don't want to make any statement."
The ministers showed little enthusiasm for proposals, reportedly pushed by Britain and France, to partially lift a weapons embargo on Syria to help arm rebels fighting to oust al-Assad.
Syria's main opposition group, the National Coalition of Syrian Revolutionary and Opposition Forces, was to meet in Cairo Thursday, a day after its chief Moaz al-Khatib said he was willing to hold talks with officials representing the Syrian regime.
Al-Khatib's statement drew mixed reaction amid unconfirmed reports that some opposition politicians have called for his resignation.
The opposition has frequently rejected any talks with the Syrian regime unless al-Assad leaves power.