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 jihadist forums.
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.
The joint group would be willing to ally with other organisations "on the condition that the country and its citizens be governed according to the rules dictated by Allah," he added.
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 an audio message posted online on Sunday.
"Let your fight be in the name of Allah and with the aim of establishing Allah's sharia (law) as the ruling system," said Zawahiri. "Do all that you can so that your holy war yields a jihadist Islamic state."
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 battlefronts across Syria.
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 Al-Qaeda in Iraq.
At the time, the US State Department described it as a "new alias" for Al-Qaeda in Iraq, and said it 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.
Officially, Al-Nusra was established in January 2012, according to a video statement declaring its creation. It 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.
No group claimed responsibility for Monday's bombing in Damascus, but the opposition National Coalition condemned the attack, while suggesting Assad's 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," the group said in a statement on Tuesday.
Syria's conflict, now in its third year, is believed to have killed more than 70,000 people since it erupted.
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.