Al-Qaida’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 already claimed responsibility for Sunday’s assault on a Catholic church 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-Qaida in Iraq and other allied Sunni insurgent factions. It is unclear exactly what led the group to seize on conversion disputes between Egypt’s Muslims and its minority Christians.

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-Qaida-linked prisoners held in Iraq.

“All Christian centers, organizations and institutions, leaders and followers are legitimate targets for the muhajedeen (holy warriors) wherever they can reach them,” it said.

The group specifically mentioned two Egyptian women married to Coptic priests, whom some believe converted to Islam to leave their husbands as 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 conversion issue has become a rallying point for hard-line Islamists in Egypt.

The Baghdad church siege horrified Iraq’s Christian community, hundreds of whom gathered Tuesday for a memorial service in Baghdad. One of the officials read a letter from Pope Benedict XVI to the crowd.

“For years the violence hasn’t stopped hitting this country, and Christians are becoming the target of these cruel terrorist attacks,” the letter read.

Sunday’s attack was the deadliest ever recorded against Iraq’s Christians, whose numbers have plummeted since the 2003 U.S.-led invasion as the community has fled to other countries.

On Tuesday, it was Iraq’s Shiite Muslims who bore the brunt of violence after 13 attacks struck neighborhoods across the capital.

The death toll in that violence climbed to 91 people by Wednesday, according to Iraqi police and hospital officials. No breakdown of the new death toll was immediately available. All officials spoke on condition of anonymity because they were not authorized to talk to the press.

Those attacks evoked painful memories of the bloody sectarian war between Iraq’s Sunni and Shiite militias fought in 2006 and 2007, which killed tens of thousands of innocent civilians.

The bombings hit civilians at restaurants and cafes where many Iraqis gather to enjoy the balmy evenings. The attacks demonstrated the insurgents’ ability to still carry out coordinated strikes from one end of Baghdad to the other despite a network of police and army checkpoints and blast walls crisscrossing the capital.

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 injured in attacks were a first for al-Maliki since he took office in 2006 — the year the country broke down along sectarian lines, prompting tit-for-tat killings between Sunnis and Shiites, driving millions of Iraqis from their homes and the country.

Al-Maliki 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 prolonged contentious fight for allies that has left the government stalemated and Iraq’s nascent political process deadlocked.

A new government still seems weeks, if not month away, though there was a glimmer of hope for progress when parliament’s acting speaker, Fouad Massoum, called the lawmakers to convene on Monday and elect his successor.

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 and then later 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.