BEIRUT, Lebanon — Iraq’s branch of Al Qaeda said Tuesday that it had merged with the Nusra Front, a group of jihadists fighting to topple President Bashar al-Assad of Syria, in a marriage that appeared to strengthen the role of Islamic militants in the Syrian insurgency and further complicate Western assistance efforts.

The United States has already blacklisted the Nusra Front over evidence of its links with the Islamic State of Iraq, the Qaeda branch. But the news of the merger, made by the branch’s leader, Abu Bakr al-Baghdadi, in an audio statement posted on jihadist Web sites, was the first time he had announced that they were a single organization.

“The time has come for us to announce to the people of the Levant and to the whole world that Al Nusra Front is merely an extension of the Islamic State of Iraq and a part of it,” Mr. Baghdadi said. He also said the combined group would be called the Islamic State in Iraq and the Levant and would dedicate half its budget to the Syrian insurgency.

In a warning to other Syrian fighters who want Mr. Assad to go but do not share Mr. Baghdadi’s views, he said, “Don’t make democracy a price for those thousands among you who have been killed.”