Profile: Syria's al-Nusra Front
The al-Nusra Front's pledge of allegiance to al-Qaeda has ended speculation over the suspected ties between the Syrian jihadist group and the Islamist militant network.
The announcement came just days after al-Qaeda's leader, Ayman al-Zawahiri, called on jihadis to do everything possible to bring about an Islamic state in Syria.
But al-Nusra was quick to stress that the oath would have no impact on its role in Syria, where it has come to play a significant role in the fight against the government of President Bashar al-Assad.
The Front's leading figure, Abu Mohammed al-Jawlani, assured Syrians that the "good behaviour" they had experienced from al-Nusra on the ground would continue unchanged.
He also rejected claims that al-Nusra had merged with al-Qaeda's Iraq branch, saying he had not been consulted on the matter.
Nevertheless, the pledge is likely to put the Front in an awkward position as it tries to win the support of the population in rebel-held areas, and to keep the goodwill of other opposition groups who do not want to be associated with al-Qaeda.
Al-Nusra - or The Front for the Defence of the Syrian People - first announced its existence with a video posted online in January last year.
In the statement, the group said it was behind many of the suicide bombings that have rocked Syria since the uprising began in March 2011.
"We are Syrian mujahideen, back from various jihad fronts to restore God's rule on the Earth and avenge the Syrians' violated honour and spilled blood," a masked man declared in the video.
"Jabhat al-Nusra has taken upon itself to be the Muslim nation's weapon in this land."
Since its first public appearance, al-Nusra is thought to have led numerous guerrilla attacks against strategic state targets, including the capture of a key airbase in the north.
The US has blacklisted the group as a terrorist organisation in response to the bombing campaigns.
Importantly, al-Nusra - which means "support" in Arabic - has developed a reputation for discipline and honesty, correspondents say.
This has helped it to gain a key role in rebel-held areas in the northern city of Aleppo, where it has taken over distribution of flour to bakeries and set up a Sharia court to administer Islamic law.
Al-Nusra's propaganda often appears designed to appeal to ordinary Muslims.
It emphasises purported efforts to avoid civilian casualties and has pictured group members speaking to attentive crowds in Syrian towns.
The Front's statements and videos are usually issued by its media group, al-Manara al-Baida (the White Minaret), and are regularly posted to jihadist, social media and video-sharing websites. There is even a Facebook page dedicated to the group.
Its videos are usually filmed in the documentary style that major jihadist groups tend to employ, and include the wills of its alleged suicide bombers, whose names all suggest that they are Syrian.
The group's leader has not appeared in person in any of its videos, preferring to feature only on audio tracks.
This secretive approach extends to concealing the identities of fighters and civilians appearing in the videos.
Al-Nusra frames its attacks as retribution for alleged atrocities committed by Syrian security forces and pro-government militia.
But it has also referred to the US and Israel as enemies of Islam, and attacked the beliefs of other religious groups in Syria, including the Alawites.
And although the group may prefer to downplay its true ambitions, the message is clear: this is not a fight for democracy, but for the creation of Syria as an Islamic state ruled under Sharia.
Alleged members of al-Nusra usually display the black flag that is associated with global jihadist groups, particularly al-Qaeda in Iraq.
Al-Nusra's connection to al-Qaeda has led the Free Syrian Army (FSA) opposition to distance itself from the movement.
"We don't support the ideology of al-Nusra," FSA spokesman Louay Meqdad said.
"There has never been and there will never be a decision at the command level to coordinate with al-Nusra."
Mr Meqdad did, however, acknowledge that that there had been co-operation between FSA brigades and the Front on "certain operations".
Al-Nusra's role in the anti-Assad fight also represents a headache for Western leaders debating whether to arm Syrian rebels.
Critics have raised concern of weapons ending up in the hands of groups such as al-Nusra.