WASHINGTON -- In a pair of speeches at the annual gathering of a major pro-Israel group, top U.S. and Israeli leaders expressed deep concerns about the civil war in Syria and indicated they worried as much about the situation that might emerge after the removal of the current regime as they have about the regime itself.

"The United States and Israel have a shared interest in Syria," said Vice President Joe Biden, during a generally warm speech before some 13,000 attendees at the annual policy conference of the American Israel Public Affairs Committee (Aipac). "[President Bashar] Assad has been engaging in the brutal murder of his own citizens, and our position in that regard cannot be clearer: Assad must go."

But Biden continued, "We are not signing up for one murderous gang replacing another in Damascus."

Although the Syrian rebels have been fighting Assad's army for two years now, the United States has been very cautious about providing them with support, in part motivated by grave concerns that any military aid could fall into the hands of rebel fighters who might use it against the U.S. or Israel. Only last week, in fact, did the U.S. finally pledge to directly supply some rebel groups with nonlethal assistance, such as food and medicine.

Biden acknowledged that the process has been gradual, saying that the U.S. focus has been on "carefully vetting those" to receive assistance. He emphasized that at the same time, the U.S. has taken steps to impede the efforts of at least one major rebel brigade, by designating it as a terrorist group for its reported links to al Qaeda in Iraq.

The possibility that the fall of the Assad regime could empower radical Islamist groups, who might gain access to Syria's rich stores of chemical and biological weapons, was also a concern voiced by Israeli Prime Minister Benjamin Netanyahu when he spoke to the Aipac crowd via satellite from Jerusalem.

"Syria could soon become a strategic crisis of unforeseen proportions," Netanyahu said, after noting that crisis would be one of three main topics for discussion when President Barack Obama visits Israel later this month.

"The danger of these weapons falling into the hands of these terrorist groups is very real," Netanyahu warned. "Terrorist groups like Hezbollah and al Qaeda are trying to seize these weapons as we speak -- they're like a pack of hyenas trying to feed off a carcass, and the carcass isn't dead yet."

Enthusiasm for the uprising in Syria has waned significantly among pro-Israel and Israeli advocates -- a trend reflected in the revolution's lower profile at this year's Aipac gathering than in the year before, when the rising aspirations of the Syrian people were celebrated at panel discussions and the general assembly.

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  • In this Sunday Jan. 6, 2013 photo Syrian rebel fighters take their positions on the frontline of the ongoing battle for the military airport in Taftanaz, Syria. (AP Photo/Mustafa Karali)

  • In this Sunday Jan. 6, 2013 photo, an unexploded ordnance lies half-buried in the ground on the frontline of the ongoing battle for the military airport in Taftanaz, Syria. (AP Photo/Mustafa Karali)

  • In this Sunday Jan. 6, 2013 photo, a Syrian rebel fighter walks through a field on the frontline of the ongoing battle for the military airport in Taftanaz, Syria. (AP Photo/Mustafa Karali)

  • In this Friday, Jan. 11, 2013 file citizen journalism image provided by Edlib News Network, ENN, which has been authenticated based on its contents and other AP reporting, shows rebels from al-Qaida affiliated Jabhat al-Nusra waving their brigade flag as they step on the top of a Syrian air force helicopter, at Taftanaz air base. (AP Photo/Edlib News Network ENN, File)

  • This citizen journalism image provided by Edlib News Network, ENN, which has been authenticated based on its contents and other AP reporting, shows a damaged Syrian air force helicopter at Taftanaz air base, in Idlib province, northern Syria, Friday Jan. 11, 2013. (AP Photo/Edlib News Network ENN)

  • In this image taken from video obtained from Ugarit News, which has been authenticated based on its contents and other AP reporting, a statue of Hafez Assad, father of Syrian President Bashar Assad, burns after being set on fire by rebel fighters inside the grounds of the General Company of the Euphrates Dam in Al-Raqqa, Syria, Monday, Feb. 11, 2013. (AP Photo/Ugarit News via AP video)

  • This image taken from video obtained from Ugarit News, which has been authenticated based on its contents and other AP reporting, shows a Syrian fighter jet in a hangar after rebels captured Jarrah airfield in Aleppo province, Tuesday, Feb. 12, 2013. (AP Photo/Ugarit News via AP video)

  • This image taken from video obtained from Ugarit News, which has been authenticated based on its contents and other AP reporting, shows a Syrian fighter jet in a hangar after rebels captured Jarrah airfield in Aleppo province, Tuesday, Feb. 12, 2013. (AP Photo/Ugarit News via AP video)

  • This photo released by the Syrian official news agency SANA, shows flames and smoke rising from burned cars after a huge explosion that shook central Damascus, Syria, Thursday, Feb. 21, 2013. (AP Photo/SANA)

  • This photo released by the Syrian official news agency SANA, shows Syrian fire fighters extinguishing burning cars after huge explosion that shook central Damascus, Syria, Thursday, Feb. 21, 2013. (AP Photo/SANA)

  • This photo released by the Syrian official news agency SANA, shows a Syrian fire fighter extinguishing burning cars after a huge explosion that shook central Damascus, Syria, Thursday, Feb. 21, 2013. (AP Photo/SANA)

  • In this picture taken on Thursday, Feb. 21, 2013, a Free Syrian Army fighter takes his position at a previous Syrian army forces checkpoint at the main entrance of Christian village of Yacoubieh, in the northern Syrian province of Idlib, Syria. (AP Photo/Hussein Malla)

  • A member of the Syrian security forces inspects the heavily damaged Zahrawi souq during a patrol in the old city of Aleppo on February 23, 2013. (STR/AFP/Getty Images)

  • Free Syrian Army fighters pray inside a cave at Jabal al-Zaweya in Idlib, Syria, Sunday Feb. 24, 2013. (AP Photo/Hussein Malla)

  • Free Syrian Army fighters take their positions as they observe the Syrian army forces base of Wadi al-Deif, at the front line of Maarat al-Nuaman town, in Idlib province, Syria, Tuesday Feb. 26, 2013. (AP Photo/Hussein Malla)

  • A Free Syrian Army fighter steps out from a hole wall to observe the Syrian army forces base of Wadi al-Deif at the front line of Maarat al-Nuaman town, in Idlib province, Syria, Tuesday Feb. 26, 2013. (AP Photo/Hussein Malla)

  • A Free Syrian Army sniper takes his position as he observes the Syrian army forces base of Wadi al-Deif at the front line of Maarat al-Nuaman town, in Idlib province, Syria, Tuesday Feb. 26, 2013. (AP Photo/Hussein Malla)

  • Free Syrian Army fighters from the Knights of the North brigade move to reconnaissance a Syrian army forces base of al-Karmid at Jabal al-Zaweya, in Idlib province, Syria, Wednesday, Feb. 27, 2013. (AP Photo/Hussein Malla)

  • Free Syrian Army fighters from the Knights of the North brigade leave one of their caves to reconnaissance a Syrian army forces base of al-Karmid, at Jabal al-Zaweya, in Idlib province, Syria, Wednesday, Feb. 27, 2013. (AP Photo/Hussein Malla)

  • Citizen journalism image provided by Coordination Committee in Kafr Susa which has been authenticated based on its contents and other AP reporting, shows people tearing down a huge poster of President Bashar Assad and hitting it with their shoes, in Raqqa, Syria, Monday, March. 4, 2013. (AP Photo/Coordination Committee In Kafr Susa)