Al-Qaeda's branch in Iraq says it has merged with Syria's armed opposition group Jabhat al-Nusra, a move that shows the rising confidence of hardliners within the Syrian rebel movement and is likely to stoke renewed fears among its international backers.
A website linked to Jabhat al-Nusra confirmed on Tuesday the merger with the Islamic State of Iraq, whose leader, Abu Bakr al-Baghdadi, first made the announcement in a 21-minute audio message posted on websites late on Monday.
Jabhat al-Nusra has taken an ever-bigger role in Syria's conflict over the last year, fighting in crucial battles with President Bashar al-Assad's forces and staging several large suicide bombings.
The US has designated it a terrorist organisation.
The Syrian group has made little secret of its links across the Iraqi border, but until now it has not officially declared itself to be part of al-Qaeda.
Group's new name
Baghdadi said that his group, the Islamic State of Iraq, and Syria's Jabhat al-Nusra will now be known as the Islamic State in Iraq and the Levant.
"It is time to announce to the Levantine people and the whole world that Jabhat al-Nusra is merely an extension and part of the Islamic State of Iraq," he said.
He said that the Iraqi group was providing half of its budget to the conflict in Syria.
In-depth coverage of escalating violence across Syria
Baghdadi said the Syrian group would have no separate leader but instead be led by the "people of Syria themselves", implying that he would be in charge in both countries.
The formal merger of such a prominent Syrian rebel group to al-Qaeda is likely to cause concern among backers of the opposition who are enemies of the global network, including both Western countries and Gulf Arab states.
It may increase resentment of Jabhat al-Nusra among other rebel factions.
Rebels have until now respected the hardline group's fighters for their prowess on the battlefield, but a merger with al-Qaeda will complicate any effort to send arms to rebels from abroad.
A website linked to 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 websites belonging to groups are rarely disputed by armed groups afterwards.
Disparate rebel groups
Jabhat al-Nusra emerged as an offshoot of Iraq's al-Qaeda branch in early 2012, as one of a patchwork of disparate rebel groups in Syria.
A top Iraqi intelligence official told the Associated Press news agency in Baghdad that they have always known that "al-Qaeda 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 armed 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 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.