Al-Qaeda's front group in Iraq has threatened more attacks on Christians following a bloody siege at a Baghdad church that left 58 people dead, saying the "killing sword will not be lifted" from their necks.

Iraqi security forces stand guard outside the National Evangelical church in Basra, Iraq's second-largest city (Nabil al-Jurani/Associated Press)

The Islamic State of Iraq's warning of further violence against Christians comes two days after the group's assault on a Catholic church in downtown Baghdad — the deadliest attack 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.

"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, an umbrella group that includes al-Qaeda in Iraq and other allied Sunni insurgent factions, also said its deadline for the Coptic Christian Church in Egypt to release Muslim women that the militant group claims are being held captive has expired.

As a result, "all Christian centres, organizations and institutions, leaders and followers, are legitimate targets for the muhajedeen [holy warriors] wherever they can reach them," the group said. The statement did not specify any one location, raising the spectre of violence against Christians across the region.

The release of the women in Egypt was one of the militants' demands during Sunday night's siege, along with the release of al-Qaeda-linked prisoners held in Iraq.

The militants have specifically mentioned two Egyptian women who are married to Coptic priests. Some believe they converted to Islam to leave their husbands since divorce is banned by Egypt's Coptic Church.

Conversion a rallying point

Over the past few years in Egypt, arguments over conversions in both communities have worsened tensions already high over issues like the construction of new churches. The two communities generally live in peace, though clashes and attacks 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.

While Christians were the target Sunday night, Shias bore the brunt of a string of 13 attacks on Tuesday that struck neighbourhoods across the capital.

An Iraqi police officer stands at the scene of a car bomb attack in front of an Assyrian Catholic Church in Baghdad on Monday. An al-Qaeda group has threatened more violence against Christians. (Khalid Mohammed/Associated Press)

On Wednesday, the death toll in that violence climbed to 91 people, 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.

The bombings hit civilians at restaurants and cafes where many Iraqis were gathered to enjoy the warm evening. The violence demonstrated the insurgents' ability to carry out co-ordinated attacks from one side 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 bedside calls 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 of Sunnis and Shias and driving millions of Iraqis out of their homes and out of the country.

Al-Maliki has been struggling to keep his job since his Shia-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.