The leader of the most formidable rebel group in Syria pledged allegiance Wednesday to al-Qaida, but distanced himself from a claim that his Islamic extremist faction had merged with the terror network's Iraqi branch.

Al-Qaida in Iraq said Tuesday that it had joined forces with Jabhat al-Nusra, or the Nusra Front, the most effective force among the disparate rebel factions fighting to topple Syrian President Bashar Assad. It said they had formed a new alliance called the Islamic State in Iraq and the Levant.

Talk of an alliance raised fears in Iraq, where intelligence officials said recently that increased cooperation was already evident in a number of deadly attacks. And in Syria, a stronger Nusra Front would only further complicate the battlefield where Western powers have been trying to funnel weapons, training and aid toward more secular rebel groups and army defectors.

But the leader of Nusra Front, Abu Mohammad al-Golani, threw doubt on the merger. In a statement posted on militant websites, he said he was not consulted ahead of time and only heard about the union through the media.

He did not, however, deny the two groups had merged, and remained vague on the point, only saying that the announcement was premature. He said his group will continue to use Jabhat al-Nusra as its name.

"The banner of the Front will remain unchanged despite our pride in the banner of the State and those who carried it and sacrificed and shed their blood for it," he said in a reference to al-Qaida in Iraq.

The message appeared to be, at least in part, an effort by Nusra Front to reassure Syrians that the group remains dedicated to the uprising to oust Assad and is not beholden to non-Syrian interests despite its pledge of fealty to al-Qaida leader Ayman al-Zawahri.

"What you saw from the Front of its defense of your religion, honors, and blood, and its good qualities with you and the fighting groups, will remain as you experienced it," al-Golani said in remarks addressed to the Syrian people.

"The announcement of the pledge of allegiance will not change anything in [Nusra's] policy."

Earlier this week, al-Zawahiri urged Islamic fighters in Syria to unite in their efforts to oust Assad. That may have provided at least part of the impetus for the announced merger with al-Qaida in Iraq, formally known as the Islamic State in Iraq.

The purported unification was announced by ISI leader Abu Bakr al-Baghdadi in a 21-minute audio message posted on militant websites late Monday.

In his recording, al-Golani confirmed his group's long-standing, close ties with al-Qaida's Iraqi franchise, and expressed gratitude for what the money and manpower it provided to help get Jabhat al-Nusra off the ground.

Al-Golani's message was first reported by the SITE Intelligence Group, which monitors extremist websites.

The Syrian government seized upon the announced merger to back its assertions that it is not facing a true popular movement for change but rather a foreign-backed terrorist plot.

The state news agency said Wednesday that the union "proves that this opposition was never anything other than a tool used by the West and by terrorists to destroy the Syrian people."

It added that the merger "puts the credibility of the United Nations and the Security Council and independent countries in front of a real test in which they must choose between their bias for terrorism represented by al-Qaida ... or acknowledge the Syrian people and government's right to combat terrorism."

The U.S.­ designated Nusra Front a terrorist organization in December, and said then that the group had ties to al-Qaida. The Syrian group's public oath of allegiance to al-Qaida is unlikely to prompt a shift in international support for the broader Syrian opposition.

The U.S.­ and its allies are already working to try to counter the rising influence of Nusra Front and other Islamic extremists in the civil war by boosting their support for rebel factions deemed to be more moderate.

On Wednesday, U.S.­ Secretary of State John Kerry met with Syrian opposition leaders in London to discuss ways to step up aid to rebels.

So far, the U.S.­ and its allies have helped create 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.

Nusra Front, which has welcomed militants from across the Muslim world into its ranks, has made little secret of its links across the Iraqi border. The 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.