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 centres, organizations and institutions, leaders and followers are legitimate targets for the mujahedeen (holy warriors) wherever they can reach them," it said.

The group specifically mentioned two Egyptian women married to Coptic priests 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 past few years in Egypt, arguments over these kinds of alleged conversions have exacerbated Muslim-Christian tensions already high over issues like the construction of new churches. The two communities generally live in peace, though clashes have taken place.

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 Tuesday 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 press.

Iraqi state TV aired footage Wednesday of Prime Minister Nouri al Maliki visiting victims of the blasts in Baghdad's hospitals. The televised trips to civilians wounded in attacks were a first for al Maliki, who has been struggling to keep his job since his Shiite-dominated alliance was narrowly defeated by the Sunni-backed bloc of former Prime Minister Ayad Allawi in the March 7 parliamentary election.

Neither bloc won an outright majority, setting up a fight for allies that has left the government stalemated. There was a glimmer of hope for political progress Wednesday when parliament's acting speaker, Fouad Massoum, called the lawmakers to convene Monday and elect his successor.

However, the acting speaker only has the right to call parliament to session and can't necessarily force all the members to show so it was unclear whether the date would hold or that the announcement signified any progress in the political talks.

Last week, Iraq's highest court ordered the 325 lawmakers back to work after a virtual eight-month recess. The parliament has met only once since the March 7 vote for just 20 minutes to allow more time to choose a new leadership.

Under the constitution, parliament was required to meet within 15 days of final court approval of election results and choose a speaker, then a president. The appointments had to be put off because they are part of the negotiations over the rest of the new leadership -- including a prime minister and top Cabinet officials.