Al-Qaeda in Iraq confirmed on Tuesday long-held suspicions that Al-Nusra Front, a jihadist group fighting in Syria, is part of its network, an admission likely to sharpen debate on arming rebels.
"It is time to declare to the Levant and to the world that the Al-Nusra Front is simply a branch of the Islamic State of Iraq," ISI's chief Abu Bakr al-Baghdadi said in an audio message posted on the Internet, adding that it is fighting for an Islamic state in Syria.
The groups would be combined and called the Islamic State in Iraq and the Levant, Baghdadi said, describing Al-Nusra front leader Abu Mohammed al-Jawlani as "one of our soldiers".
"We chose al-Jawlani... as well as other fighters to go from Iraq to Syria... We prepared plans and work policies. We gave them money and personnel support," said Baghdadi.
His declaration came a day after an Al-Nusra-style suicide car bomb attack in the heart of the Syrian capital Damascus killed at least 15 people and wounded 146 others.
It also came after Al-Qaeda's global chief, Ayman al-Zawahiri, urged rebels to fight to establish an Islamic state in Syria, in a recording posted online on Sunday.
Al-Nusra Front is among the most prominent organisations involved in Syria's conflict, which erupted in March 2011 with peaceful protests against President Bashar al-Assad's regime but has evolved into a war that has killed tens of thousands.
The Islamist militant group has gained notoriety for its suicide bombings but also won admiration among some insurgents over its reputation as a formidable fighting force leading attacks on Syrian battlefronts.
The West has been wary, and in December the United States announced it was labelling Al-Nusra Front a "terrorist" organisation because of suspected ties to ISI.
At the time, the US State Department said the creation of Al-Nusra was "an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes".
According to the US, the head of Al-Qaeda in Iraq "is in control of both AQI and Al-Nusra" and reports on Internet forums used by jihadists indicate hundreds of militants have made the trip from Iraq into Syria to fight Assad's regime.
Al-Nusra, which announced its creation in a January 2012 video, is a magnet for foreign fighters seeking to take part in the uprising against the Syrian regime.
Suspicions about Al-Nusra's ties to ISI were raised by similarities in the group's tactics, including their preference for suicide and car bomb attacks.
The formal announcement of the ties could complicate the West's ongoing involvement with rebel forces battling Assad's regime.
Despite backing the uprising, Western governments have shown little enthusiasm for arming its fighters, in part for fear of the weapons falling into the hands of Al-Qaeda allied groups like Al-Nusra.
The New York Times reported last month the CIA was helping Arab states and Turkey to boost arms shipments to Syria's rebels, in a move to ensure the weapons do not end up in the hands of extremists.
In March, France and Britain argued that scrapping an EU arms embargo for Syria would tilt the balance on the ground and help prompt a political settlement of the conflict.
But France has since backtracked, saying it was still undecided.
No group claimed responsibility for Monday's bombing in Damascus, but the opposition National Coalition condemned the attack while suggesting the regime bore responsibility.
"The Syrian Coalition, on behalf of the Syrian people, condemns without any hesitation, all bombings and hostilities against civilians, and considers these acts crimes against humanity," said a statement.
Syria's conflict, now in its third year, is believed to have killed more than 70,000 people.
At least another 89 people -- 41 civilians, 26 rebels and 22 soldiers -- were killed across the country on Monday, said the Britain-based Syrian Observatory for Human Rights.