A group calling itself "The Battalion of Just Punishment, Jihad Base in Mesopotamia" sent a letter to churches in northern Iraq threatening the Assyrian community for collaborating with U.S. forces and for seeking to establish an autonomous police force, Assyrian International News Agency said Wednesday.

"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 target of the freedom fighters," the AINA translation of the letter read.

The Assyrian community in northern Iraq witnessed widespread persecution since the 2003 U.S.-led invasion of Iraq, particularly from Islamic militant groups such as Al-Qaida.

"We remind the dhimmi people that Iraq is for the noble Iraqis and not for how you are now," the letter said, using the Koranic term for those with fewer legal and social rights than Muslims under Shariah law.

Assyrian analysts who have seen the letter said some of its references suggest it came from Ansar Al-Islam, the Kurdish group allegedly linked to al-Qaida.