An entire 1,000-strong rebel brigade based in Syria’s Idlib province has reportedly defected to the Islamic State group, raising new questions as to whether other factions will rally behind the militant force that has swept through Iraq’s west.

The Dawud Brigade, which was based in Sarmin and fell under the umbrella of the anti-government Sham Army, arrived in the northeastern city of Raqqa last weekend, the main headquarters for the Islamic State (IS) – previously known as the Islamic State of Iraq and Syria (ISIS/ISIL).

The rebel group arrived in a convoy of over 100 vehicles, including 10 tanks seized from Syrian government forces, McClatchy reported quoting anti-government activists as saying on Tuesday. Syrian journalist Ammar Abu Shahin said that an increasingly complex relationship between anti-government rebel groups prompted Dawun Brigade’s leader, Hassan Abboud, to head to Raqqa.

“Frankly speaking, people in the countryside of Idlib are in a real panic about the advancement of the Islamic State,” said Shahin.

According to the journalist, who is familiar with the dynamics between pro-Western Syrian rebels and more radical Islamist elements, the decision by the brigade to formally align itself with the Islamic State was “only a change in uniform,” suggesting that groups already sympathetic could well identify themselves and also head for its Raqqa headquarters.

Meanwhile there have been reports on Monday that several rebel factions affiliated with the FSA have pledged allegiance to the IS in the border town of al-Bukamal in eastern Syria. Sources and eyewitnesses told Zaman Alwasl newspaper that brigades of Ahl Al Athar, Ibin al-Qa'im, and Aisha have joined the IS.

The reports of the defections come as the Obama administration ratchets up efforts to arm the Free Syrian Army. Last month the White House asked Congress for half-a-billion dollars in aid to go towards the opposition fighters.

US Defense Secretary Chuck Hagel ordered the Pentagon to develop more detailed plans to train and equip moderate Syrian rebels, should Congress approve the requested funding.

Having already declared a “caliphate,” IS is said to be planning to fuse its controlled territories in Syria with those it has claimed in Iraq, eliminating the border and securing oil and gas interests in the process. The group’s leader, Abu Bakr al-Baghdadi, has called on Muslims throughout the world to travel and join the Islamic caliphate.

Despite territorial gains in Syria, IS does not appear to have gained much support from Syrian citizens – just four percent of those recently polled believe the Islamic insurgents represent their interests.

The situation in Iraq, however, may be decisively different. There, the group has preyed upon the sectarian rift between Sunni and Shia Muslims in the country, and has been joined by seasoned members of the former Baath ruling party.