BAGHDAD - Al-Qaeda's front group in Iraq has threatened more attacks on Christians after a siege on a Baghdad church that left 58 people dead, linking the warning to claims that Egypt's Coptic Church is holding women captive for converting to Islam.

The Islamic State of Iraq, which has claimed responsibility for Sunday's assault on a Catholic church during Mass in downtown Baghdad, said its deadline for Egypt's Copts to release the women had expired and its fighters would attack Christians wherever they can be reached.

"We will open upon them the doors of destruction and rivers of blood," the insurgent group said in a statement posted late Tuesday on militant websites.

The Islamic State of Iraq is an umbrella group that includes al-Qaeda in Iraq and other allied Sunni insurgent factions.

It is unclear exactly what led the group to seize on the conversion disputes between Egypt's Muslims and its minority Christians, although the issue has become a rallying point for hard-line Islamists in Egypt.

In announcing its reasons for Sunday's attack, the group said it had given the Coptic Church 48 hours to release the women it says had converted to Islam. The group also demanded the release of al-Qaeda-linked prisoners held in Iraq.

"All Christian centers, organizations and institutions, leaders and followers are legitimate targets for the mujahideen [holy warriors] wherever they can reach them," it said.

The group specifically mentioned two Egyptian women married to Coptic priests who it says are being held against their will. The church denies the allegation. Some believe the women converted to Islam to leave their husbands because divorce is banned by the church.

Over the last few years in Egypt, arguments over these kinds of alleged conversions have exacerbated Muslim-Christian tensions already high over issues such as the construction of new churches. The two communities generally live in peace.

The Baghdad church siege was the deadliest ever recorded against Iraq's Christians, whose numbers have plummeted since the 2003 U.S.-led invasion as members of the community have fled to other countries to escape the violence.

The death toll in a series of attacks mainly targeting Shiites in Baghdad, meanwhile, rose to 91, according to Iraqi police and hospital officials, who spoke on condition of anonymity because they were not authorized to talk to the news media.