By Barbara Starr

Al Qaeda's affiliate inside Syria is now the best-equipped arm of the terror group in existence today, according to informal assessments by U.S. and Middle East intelligence agencies, a private sector analyst directly familiar with the information told CNN.

Concern about the Syrian al Qaeda-affiliated group Jabhat al-Nusra, also known as the al-Nusra Front, is at an all-time high, according to the analyst, with as many as 10,000 fighters and supporters inside Syria. The United States has designated al-Nusra Front as a terrorist group with links to al Qaeda in Iraq.

That assessment is shared by some Middle Eastern intelligence agencies that have long believed the United States is underestimating the Sunni-backed al Qaeda movement in the country, according to a Middle East source. It is also believed that Iran is running training camps inside Syria for Hezbollah and that other Iranian militia fighters are coming into the country to fight for the regime.

The analyst has been part of recent discussions with the U.S. intelligence community, which is urgently working to understand what is going on inside the war-ravaged country and is consulting outside experts. The analyst, who declined to be named because of the sensitive nature of the information, stressed that all assessments about Syria are approximate at best because of the lack of U.S. personnel on the ground.

With the growing strength and support for al-Nusra, U.S. concerns are growing about its influence to further destabilize Syria and potentially pose a greater regional threat, administration officials have told CNN.

"They are making desperate attempts to get chemical weapons," the analyst told CNN, noting that in the past few weeks, security services in Iraq and Turkey arrested operatives who were "trying to get their hands on sarin."

A senior U.S. intelligence official told CNN recently that gathering intelligence on Syria, including its potential future use of chemical weapons, is now one of the top priorities of the U.S. intelligence community.

The Obama administration announced last week that it will start arming rebels because Syria crossed a "red line" by using chemical weapons - including sarin gas - against the opposition.