Speaking two days after Israel allegedly carried out a bombing raid on a military site outside Damascus, Panetta said Washington was worried the Lebanese Shiite militia Hezbollah could exploit the 22-month conflict in Syria.
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"The chaos in Syria has obviously created an environment where the possibility of these weapons, you know, going across the border and falling into the hands of Hezbollah, has become a greater concern," said Panetta, who is due to retire this month as Pentagon chief.
The Israeli raid on Wednesday targeted surface-to-air missiles and an adjacent military complex believed to house chemical agents, according to a US official, who spoke on condition of anonymity. Syria has threatened to retaliate.
Asked about the air strike, Panetta said he could not disclose discussions with the Israelis but suggested Washington fully backed the move.
"Without discussing the communications that we have on a regular basis with Israel or the specifics of that operation, because that's something they know more about, we have expressed the concern that we have to do everything we can to make sure that sophisticated weapons like SA-17 missiles or, for that matter chemical biological weapons, do not fall into the hands of terrorists," he said.
Asked if the United States supported the Israeli action, he said: "The United States supports whatever steps are taken to make sure these weapons don't fall into the hands of terrorists."
He reiterated the United States has been working closely with Jordan, Turkey and Israel on contingency plans to ensure the security of chemical or biological weapons in Syria, particularly if the regime of President Bashar Assad collapses.
Panetta's comments on Hezbollah were the most explicit yet by a senior US official on the Shiite militia possibly securing weapons, including missiles, due to the conflict in Syria.
In the days leading up to the air strike, Israeli officials cranked up their rhetoric about Syria's weapons stockpile, which includes chemical agents, warning of dire consequences if they end up in the hands of the Iran-allied Hezbollah against which it fought a devastating war in 2006.
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