Syrian opposition leaders and rebels have criticized Assad for not responding to the airstrike, calling it proof of his weakness and acquiescence to the Jewish state.
The Syrian defense minister, Gen. Fahd Jassem al-Freij, said Israel attacked the center because rebels were unable to capture it. Al-Freij called the rebels Israel's "tools." He told the state TV, "The heroic Syrian Arab Army, that proved to the world that it is a strong army and a trained army, will not be defeated."
Ahmad Ramadan, an opposition leader, said Syria's claim that the rebels are cooperating with Israel "is an attempt by the regime to cover its weakness in defending the country against foreign aggression." He spoke by telephone from Turkey.
While Israel has remained officially silent on the airstrike, there seemed little doubt that Israel carried it out, especially given the confirmation from the U.S., its close ally.
Israel has a powerful air force equipped with U.S.-made warplanes and has a history of carrying out air raids on hostile territory. In recent years, Israel has been blamed for an air raid in Syria in 2007 that apparently struck an unfinished nuclear reactor and an arms convoy in Sudan believed to be delivering weapons to Hamas.
Israel has not confirmed either raid, but military officials routinely talk about a "policy of prevention" meant to disrupt the flow of arms to its enemies.
In the days preceding the airstrike, the Israeli warnings were heightened. Prime Minister Benjamin Netanyahu issued a series of dire comments about the threat posed by Syria's weapons.
Israel considers any transfer of these advanced weapons to be unacceptable "game changers" that would change the balance of power in the region.
Israel has grown increasingly jittery as the Arab Spring has swept through the Middle East, bringing with it a rise of hostile Islamist elements. While Assad is a bitter enemy, Israel's northern front with Syria has remained quiet for most of the past 40 years.
If Assad is toppled, the threat of al-Qaida forces operating along Israel's frontier with Syria would pose a new and unpredictable threat. Israel has been racing to reinforce its fences along its northern frontiers with Lebanon and Syria.
In addition, Israel fears that its archenemy Iran, the close ally of Syria and Hezbollah, is moving closer to developing a nuclear weapon.
Israeli leaders have vowed to prevent Iran from obtaining nuclear arms, making veiled threats to use force if international diplomacy and sanctions fail.
Israeli defense officials tried to play down Barak's comments, saying that he was voicing a general policy that Israel is ready to defend its interests and not discussing a specific incident. They also noted that he was not speaking in his native Hebrew.
Even so, it seemed that Barak, a former prime minister, military chief of staff and regular participant on the world stage, was sending a message that Israel's warnings are not hollow and that further military action should not be ruled out.
"There is a real danger now that seriously problematic weapons will reach Hezbollah, and Israel is trying to prevent this," said Reuven Pedatzur, a defense analyst at Tel Aviv University. He said the threat has reached the point "where weapons are actually being loaded on trucks and sent on their way. That is new."
Pedatzur said the decision by Syria to try to move weapons to Lebanon could indicate that Assad's days are numbered. Assad may fear that he won't be able to secure the weapons for much longer, or may be under pressure from Iran to transfer the arms to Hezbollah before he is toppled.
Israel and Hezbollah fought a monthlong war in mid-2006 that ended in a stalemate, and Israeli military planners believe it is just a matter of time before another war breaks out.
Israel says Hezbollah has already restocked its arsenal with tens of thousands of rockets and missiles, and that obtaining chemical weapons or the advanced Russian-made anti-aircraft missiles would severely hinder Israel's ability to operate in Lebanon.
In Beirut, the Lebanese military issued a statement saying that six Israeli warplanes flew over different areas of the country on Sunday.
Federman reported from Jerusalem. Associated Press writers Bassem Mroue in Beirut, Ian Deitch in Jerusalem, and Christopher Torchia in Istanbul contributed.
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