BEIRUT (AP) -- Al-Qaida's branch in Iraq and the most powerful rebel extremist group in Syria have officially joined ranks against President Bashar Assad to forge a potentially formidable militant force in the Middle East.
The merger of the Islamic State in Iraq and Jabhat al-Nusra forms a new entity that could be an even stronger opponent in the fight to topple Assad and become a dominant player in what eventually replaces his regime.
The new group, called the Islamic State in Iraq and the Levant, underscores the growing confidence and muscle of Islamist radicals fighting on the rebel side in Syria's civil war. It also bolsters the Syrian government's assertions that the regime is battling terrorists and that the uprising is a foreign-backed plot.
While the U.S. and its European and Gulf allies are concerned about the rising prominence of Islamists among the rebels, the merger is unlikely to prompt a shift in the international support. Late last year, Washington declared that Jabhat al-Nusra had ties to al-Qaida and designated it a terrorist organization.
To try to counter the rising influence of Jabhat al-Nusra and other Islamic extremists in the civil war, the U.S. and its allies have boosted their support for rebel factions deemed to be more moderate.
On the political front, they helped created the opposition umbrella group, the Syrian National Coalition, in the hope that it will serve as the united face of those trying to unseat Assad and administer much of the territory in northern Syria that rebels have managed to pry away from regime forces in the past year.
The U.S. and other countries also have stepped up covert support for rebels on the ground by helping to coordinate shipments of new weapons and training rebels in Jordan, officials say. Those receiving training are mainly secular Sunni Muslim tribesmen from central and southern Syria who once served in the army and police.
The force is seen as a counterbalance to the Islamic militant groups -- chief among them Jabhat al-Nusra -- that have proven to be among the most effective of the myriad rebel factions fighting Assad's forces, officials say.
The merger was announced by the leader of the Islamic State of Iraq, Abu Bakr al-Baghdadi, in a 21-minute audio message posted on militant websites late Monday. A website linked to Jabhat al-Nusra known as al-Muhajir al-Islami -- the Islamic emigrant -- confirmed the merger.
Together, the groups will now be known as the Islamic State in Iraq and the Levant, al-Baghdadi said. The Levant is the traditional name referring to the region from southern Turkey to Egypt on the eastern Mediterranean.
"It is time to announce to the Levantine people and the whole world that Jabhat al-Nusra is merely an extension and part of the Islamic State of Iraq," he said.
He said the Iraqi group was providing half of its budget to the conflict in Syria, and that Jabhat al-Nusra would not have a separate leader but instead be led by the "people of Syria themselves" -- implying that he would be in charge in both countries.
The announcement comes two days after the leader of al-Qaida, Ayman al-Zawahiri, urged Islamic fighters in Syria to unite in their efforts to oust Assad.
Jabhat al-Nusra, which has welcomed militants from across the Muslim world into its ranks, has made little secret of its links across the Iraqi border, but until now it has not officially declared itself to be part of al-Qaida.
The Syrian group, which wants to oust Assad and replace his regime with an Islamic state, first emerged in a video posted online in January 2012. Since then, it has demonstrated its prowess -- and ruthlessness -- on the battlefield.
It has claimed responsibility for many of the deadliest suicide bombings against Syrian government institutions and military facilities. The group's success helped fuel a surge in its popularity among rebel fighters, although it has also emerged as a source of friction with more moderate and secular brigades in Syria.
The group also has tried to provide basic services in the parts of northern Syria under rebel control, including security and food to civilians struggling to survive.
Bilal Saab, director of the Institute for Near East and Gulf Military Analysis, North America, said the merger will make Jabhat al-Nusra an even more formidable force in Syria.
"I think you're going to have a pooling of resources, a more massive influx of fighters coming from Iraq into Syria. It will help them better position themselves in the period after Assad. It will certainly better help them defeat the Syrian regime more quickly," Saab said.