Anger continues to simmer in the Middle East as the apology from the Pope Monday for comments he made on Islam failed to bring calm.

Effigies of the Pope have been burnt in Iraq and now a militant group linked to al-Qaeda has vowed to pursue a holy war as it promised to assassinate Pope Benedict XVI.

Security at St Peter's Square remained on high alert after the promise of the militants to "conquer Rome" was made public.

The Mujahidin Shura Council, an umbrella group for Sunni Islamists, said in a statement: "We tell the worshipper of the Cross (the Pope) that you and the West will be defeated . . . May God enable us to slit their throats, and make their money and descendants the bounty of the mujahidin."

Another militant group in Iraq, Ansar al-Sunnah, added: "The day is coming when the armies of Islam will destroy the ramparts of Rome."

Meanwhile, papal nuncios (ambassadors) in Muslim countries have been instructed to reiterate this position to political and religious leaders, The Times reported, as further indication of full-scale efforts by the Vatican to bring the situation to a peaceful conclusion.

Yesterday, the Archbishop of Canterbury stepped in to defend the Pope, saying "There are elements in Islam that can be used to justify violence, just as there are in Christianity and Judaism." He added: "The Pope has issued an apology, and I think his views on this need to be judged against his entire record, where he has spoken very positively about dialogue."

The apology from the Pope has received a mixed response from leaders in the Middle East.

While the supreme leader of Iran, Ali Khamenei, said the comments were "links" in a US-Israeli conspiracy aimed at creating conflict between religions, Jordan praised the Pope for "quickly" expressing his personal regrets over his remarks, which linked Islam and violence.

Jordan added, however, that more steps were necessary to contain Muslim anger.

The Pope is expected to elaborate further on Wednesday about the comments he made last week.