An al-Qaida-affiliated militant group in Iraq has united with one of Syria's most-feared Islamic opposition groups.

The group known as the Islamic State of Iraq also revealed its formative role in creating al-Nusra Front, or Jabhat al-Nusra, a Syrian group conceived last year that the U.S. has designated as a terrorist organization, according to an announcement posted on militant websites late Monday.

"The time has come to declare before the people of Syria and the whole world, that Jabhat al-Nusra is nothing more than an extension of the Islamic State of Iraq, and a part of it," said the purported voice of Islamic State of Iraq leader Abu Bakr al-Baghdadi in an audio message.

The message said the Iraqi and Syrian groups had dropped their old names and were instead calling themselves "the Islamic State in Iraq and Syria."

Al-Nusra is one of the most feared and effective fighting groups in the Syrian uprising. The group's leadership role in rebel-held areas around northern and eastern Syria, however, has raised concerns that Assad's government will be replaced by a Sunni extremist government tied to al-Qaida.

Some Syrian rebel fighters not affiliated with al-Nusra spoke with open hostility Tuesday about the group.Abu Azzam, an opposition fighter, said other Syrian rebel groups were uniting against al-Nusra "so as to stop this cancer."