It may increase resentment of Jabhat al-Nusra among other rebel groups. Rebels have until now respected Nusra fighters for their prowess on the battlefield but a merger with al-Qaida will complicate any effort to send them arms from abroad.
A website linked with Jabhat al-Nusra known as al-Muhajir al-Islami — the Islamic emigrant — confirmed the merger.
The authenticity of neither message could be independently confirmed, but statements posted on major militant websites are rarely disputed by militant groups afterward.
Jabhat al-Nusra emerged as an offshoot of Iraq's al-Qaida branch in mid-2012 as one of a patchwork of disparate rebel groups in Syria.
One of the most dramatic attacks by the groups came on March 4, when 48 Syrian soldiers were killed in a well-coordinated ambush after seeking refuge across the border in Iraq following clashes with rebels in their home country. The attack occurred in Iraq's restive western province of Anbar, where al-Qaida is known to be active.
A top Iraqi intelligence official told The Associated Press in Baghdad that they have always known that "al-Qaida in Iraq is directing Jabhat al-Nusra."
He said they announced their unity because of "political, logistical and geographical circumstance." The official said Iraqi authorities will take "strict security measures to strike them."
Iraqi officials say the jihadi groups are sharing three military training compounds, logistics, intelligence and weapons as they grow in strength around the Syria-Iraq border, particularly in a sprawling region called al-Jazeera, which they are trying to turn into a border sanctuary they can both exploit. It could serve as a base of operations to strike either side of the border.
Baghdad officials said last week they have requested U.S. drone strikes against the fighters in Iraqi territory. A U.S. official confirmed that elements within the Iraqi government had inquired about drone strikes. But the official said the U.S. was waiting to respond until the top level of Iraqi leadership makes a formal request, which has not happened yet.