An Islamic group calling itself "The Battalion of Just Punishment, Jihad Base in Mesopotamia" sent several threatening letters to Christian churches based in areas near the northern city of Mosul. The letters warn against collaborating with the United States in Iraq, Chaldean.org reported.

"We caution and warn anyone who tries to rob us through dealings with the Americans or through the spreading of American forces and/or police to protect the Holy Shrines in the Islamic Republic of Iraq, that these shrines would remain the target of the freedom fighters," the letter said.

"We don't cooperate with anyone. They use this as an excuse to attack, torture, hold for ransom and kill innocent people," said local parishioner Khalid Bunni.

The report says Kurdish rebels are cooperating with al-Qaida elements in northern Iraq not only to drive Christians from the region but also to prevent them from joining Iraqi national security forces.

Turkish media reported Thursday intelligence officials thought al-Qaida has used the rebel Kurdistan Workers' Party as a "subcontractor" in the region.