The leader of a militant group fighting on the side of Syria's rebels has for the first time pledged allegiance to al-Qaeda and the militant network's No. 1, Ayman al-Zawahri.

In an audio message Wednesday, Abu Mohammad al-Golani also confirms ties with al-Qaeda's Iraq branch but says that he wasn't consulted ahead of an announcement on the two groups' merger.

Al-Qaeda's branch in Iraq on Tuesday announced it joined forces with the Syrian group, and that the new union will be called the Islamic State in Iraq and the Levant.

The Levant is the traditional name referring to the region from southern Turkey to Egypt on the eastern Mediterranean.

But al-Golani says the announcement was premature and that his Jabhat al-Nusra will continue to use its own name. It's unclear if al-Golani is denying the merger.

'A Syria controlled in whole or part by al-Qaeda and its affiliates — an outcome that grows more likely by the day — would be more dangerous to both our countries than anything we've seen up to now'—Iraqi Prime Minister Nouri al-Maliki

His message was first reported by the SITE monitoring service for militant groups.

Al-Golani said that the Iraqi group was providing half of its budget to the conflict in Syria. Al-Baghdadi said that the Syrian group would have no separate leader but instead be led by the "people of Syria themselves" — implying that he would be in charge in both countries.

The formal merger of such a high-profile Syrian rebel group to al-Qaeda is likely to spark concerns among backers of the opposition who are enemies of the global terror network, including both Western countries and Gulf Arab states. It may also increase resentment of Jabhat al-Nusra among other rebel factions.

U.S. Secretary of State John Kerry is currently in London meeting with Syrian opposition leaders to discuss ways to step up aid to rebels.

Influx of better weapons to rebels

Jabhat al-Nusra emerged as an offshoot of Iraq's al-Qaeda branch in early 2012, as one of a patchwork of disparate rebel groups in Syria. One of the most dramatic attacks by the group came on March 4, when 48 Syrian soldiers were killed in a well-coordinated ambush after seeking refuge across the frontier in Iraq following clashes with rebels on the Syrian side of the border. The attack occurred in Iraq's restive western province of Anbar, where al-Qaeda is known to be active.

A top Iraqi intelligence official in Baghdad said that they have always known that "al-Qaeda in Iraq is directing Jabhat al-Nusra." He said they announced their unity because of "political, logistical and geographical circumstance." The official said Iraqi authorities will take "strict security measures to strike them."

In an editorial published Tuesday in the Washington Post, Iraq's Prime Minister Nouri al-Maliki warned that a "Syria controlled in whole or part by al-Qaeda and its affiliates — an outcome that grows more likely by the day — would be more dangerous to both our countries than anything we've seen up to now."

Meanwhile, Syrian President Bashar Assad's troops battled rebels in the outskirts of Damascus on Wednesday and pressed on with a counteroffensive against opposition fighters in the south to prevent their advance on the capital.

With the recent influx of better weapons and other foreign aid, the rebels have made major gains in the south, seizing military bases and towns in the strategically important region between Damascus and the border with Jordan, about 160 kilometers from the capital.

In their campaign to topple Assad, the opposition fighters hope to eventually storm Damascus from the south.

The Britain-based Syrian Observatory for Human Rights said Wednesday's clashes were focused on opposition strongholds around the capital, including the suburbs of Daraya and Harasta. Fighting also raged in and around the northern city of Aleppo, Syria's largest city and its main commercial hub, they said.

The activists said both sides sustained losses on the two battlefields. Damascus suburbs and Aleppo have been the scene of major urban warfare in Syria's two-year uprising against Assad's rule. The revolt started as peaceful protests inspired by other Arab Spring uprisings but later descended into civil war.

Kerry meets with Syrian opposition

In London, Kerry was meeting with the opposition's interim prime minister, Ghassan Hitto, and other senior figures from the Syrian National Coalition.

The Western-backed alliance has been marred by severe divisions in its ranks since its formation late last year in Qatar, and its leaders are mostly seen as disconnected from the myriad rebel forces fighting inside Syria.

The coalition said the candidates are not allowed to have ties to the Assad regime and must be advocates or supporters "of the Syrian revolution."

In London, Kerry will have a one-on-one meeting with Russian Foreign Minister Sergey Lavrov later Wednesday. Russia has been Assad's staunch ally, supplying Damascus with weapons, shielding the regime from tougher UN sanctions and supporting Assad in his resolve to remain in power.

During his stop in Israel, Kerry said the Obama administration was holding intense talks on how to boost aid to Syria's rebels and that it was important to increase pressure on Assad's regime to get it to the negotiating table.