The leader of the al-Nusra Front, a jihadist group fighting in Syria, has pledged allegiance to the leader of al-Qaeda, Ayman al-Zawahiri.
Abu Mohammed al-Jawlani said the group's behaviour in Syria would not change as a result.
Al-Nusra claims to be have carried out many suicide bombings and guerrilla attacks against state targets.
On Tuesday, al-Qaeda in Iraq announced a merger with al-Nusra, but Mr Jawlani said he had not been consulted on this.
Al-Nusra has been designated as a terrorist organisation by the US.
Debates among Western leaders over whether to arm Syria's rebels have often raised the concern of weapons ending up in the hands of groups such as al-Nusra.
"The sons of al-Nusra Front pledge allegiance to Sheikh Ayman al-Zawahiri," Mr Jawlani said in a recording released on Wednesday.
But Mr Jawlani said al-Nusra had not been consulted on the merger with al-Qaeda in Iraq and insisted his group would not change its stance in Syria.
The al-Nusra statement assured Syrians that the "good behaviour" they had experienced from the front on the ground would continue unchanged, the BBC's Jim Muir reports from neighbouring Lebanon.
Mr Jawlani said that the oath of allegiance to Zawahiri "will not change anything in its policies", our correspondent adds.
Abu Bakr al-Baghdadi, the head of the Islamic State in Iraq, which is the Iraqi wing of al-Qaeda, had said on Tuesday that his group would be joining with al-Nusra under the name The Islamic State in Iraq and the Levant.
But Mr Jawlani said he only heard about the announcement from media and had no prior knowledge of it.
Even though Mr Jawlani said in the statement that al-Nusra had received help from the Islamic State in Iraq group, correspondents say al-Nusra clearly does not want to highlight those ties.
Al-Nusra is seen as trying to win the support of the population in rebel-held areas, and also to keep the goodwill of the other opposition groups who do not want to be associated with al-Qaeda.
Spokesmen for the Free Syrian Army, considered the main armed opposition group in Syria, reacted to Wednesday's statement by distancing themselves from al-Nusra.
"We don't support the ideology of al-Nusra," FSA spokesman Louay Meqdad told the AFP news agency.
"There has never been and there will never be a decision at the command level to coordinate with al-Nusra," Mr Meqdad went on, while admitting that there had been co-operation between FSA brigades and al-Nusra on "certain operations".
Also on Wednesday, British Foreign Secretary William Hague said the crisis in Syria will be at the "top of the agenda" when G8 foreign ministers meet in London this week.
Activist groups reported that more than 70 people had died in violence in Syria on Wednesday. More than 60,000 people are estimated to have died since the uprising against the government of President Bashar al-Assad began in March 2011.