"At this point the intelligence has really kind of leveled off," he said. "We haven't seen anything new indicating any aggressive steps to move forward in that way."
Asked whether he believed Assad was heeding Western warnings against using chemical weapons, Panetta said: "I like to believe he's got the message. We've made it pretty clear. Others have as well."
He noted that the Assad regime is coming under increasing pressure from rebel forces.
"Our concern is that if they feel like the regime is threatened with collapse, they might resort to these kinds of weapons," he said.
Syria is believed to have a formidable arsenal of chemical weapons, including sarin and mustard gas, although its exact dimensions are not known. Syria is not a signatory to the 1997 Convention on Chemical Weapons and thus is not obliged to permit international inspection.
The government in Damascus has been careful not to confirm it has chemical weapons, while insisting it would never use such weapons against its own people.
"Syria doesn't own any internationally banned weapons, whether chemical, nuclear or biological," Syrian Information Minister Omran al-Zoubi told Al-Manar TV, a station owned by the Lebanese militant group Hezbollah, which is a Syrian ally. "Even if Syria possessed such weapons, it will not use them for moral reasons."
He said Western statements are similar to those that preceded the 2003 invasion of Iraq that accused Saddam Hussein of hiding weapons of mass destruction. After the U.S.-led invasion, no such weapons were found.
The Obama administration is getting ready to tighten its ties to Syria's main opposition group, the newly formed Syrian National Coalition for Opposition and Revolutionary Forces, at an international conference on the crisis in Morocco this week. The move will pave the way for greater U.S. support for those seeking to oust Assad while the administration tries to blunt the influence of extremists.
Jabhat al-Nusra is a shadowy group with an al-Qaida-style ideology whose fighters come from Libya, Tunisia, Egypt, Saudi Arabia, Iraq, the Balkans and elsewhere. Many are veterans of previous wars who came to Syria for what they consider a new "jihad" or "holy war" against Assad.
But several hundred fighters from Jabhat al-Nusra — Arabic for "the Support Front" — have also been a valued addition to rebel ranks in the grueling battle for control of Aleppo. The group also has claimed responsibility for suicide bombings on Syrian government targets.
Jabhat al-Nusra is the largest grouping of foreign jihadis in Syria, and the rebels say they number about 300 fighters in Aleppo, as well as branches in neighboring Idlib province, the city of Homs and Damascus. U.S. and Iraqi officials also have said they believe members of al-Qaida's branch in Iraq have crossed the border to join the fight against Assad.
Also Tuesday, the U.N. refugee agency said the number of Syrian refugees registered by the United Nations in the Middle East and North Africa has surpassed half a million.
The figure is climbing by more than 3,000 per day, UNHCR said. According to UNHCR's latest figures from Lebanon, Jordan, Iraq, Turkey and North Africa, more than 500,000 Syrians are either already registered or in the process of being registered.
Associated Press National Security Writer Robert Burns contributed to this report from Kuwait City. AP writer Barbara Surk contributed to this report from Sidon, Lebanon.
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