Al-Qaeda in Iraq denounced Pope Benedict XVI's visit to Turkey, calling it part of a "crusader campaign" against Islam.
The Vatican said that Wednesday's comments showed the need to fight "violence in the name of God."
The trip is Benedict's first visit to an Islamic country as pontiff, seeking dialogue with Muslims who were angered over a speech he made in September in which he cited a medieval text that linked Islam and violence.
Al-Qaeda in Iraq, the most feared Islamic militant group in Iraq, issued its statement on an Islamic militant website it often uses to post messages.
"The Pope's visit, in fact, is to consolidate the crusader campaign against the lands of Islam after the failure of the crusader leaders (in Iraq and Afghanistan) ... and an attempt to extinguish the burning ember of Islam inside our Turkish brothers," it said.
The statement did not include any direct threat to the pope, saying only that the group is "confident in the defeat of Rome in all parts of the Islamic world."
The Vatican said on Wednesday that the Al-Qaeda in Iraq denunciation shows the need for faiths to fight "violence in the name of God."
"This type of message shows once again the urgency and importance of a common commitment of all forces against violence," Vatican spokesman Rev Federico Lombardi said.
"It also shows the need of various faiths to say 'no' to violence in the name of God."
Turkey holds a significant place in the mind-set of Al-Qaeda and militant Islamic groups that sympathise with it.
Under the Ottomans, Turkey was the seat of the Islamic caliphate, the religious rule extending over much of the Islamic world until its collapse in the early 20th Century.
Al-Qaeda has said its goal is to restore the caliphate, to unify Muslims in the fight against the West.
The authenticity of the Al-Qaeda statement could not be confirmed. It was signed by the "Islamic state in Iraq," the so-called Islamic government that the group declared earlier this year and that now issues all its messages.
The group said the pope was visiting in order to ensure that Turkey -- "once a stronghold of Islam" -- remains secular and "is driven into the arms of the European Union to stop the spread of Islam."
It pointed to the pope's "bald-faced attack on Islam in September." The pope's speech at the time prompted outrage across much of the Islamic world, after he quoted a medieval document that called the Prophet Muhammad violent.
The Al-Qaeda statement came hours before US President George W Bush arrived in Jordan for talks with Iraqi Prime Minister Nouri al-Maliki, trying to find a solution to bloodshed that many believe has spiraled out of control in Iraq.
Al-Qaeda in Iraq said, "The crusader campaigns have broken against the rocks of the holy warriors ... in Iraq and Afghanistan.
The American administration and its dogs in al-Maliki's government are living the worst days of their lives."
Chaos has increased in Iraq since Sunni insurgents set off a string of car bombs last week, killing more than 200 people in a Shiite neighborhood of Baghdad.
The United States President George W Bush this week accused Al-Qaeda of fueling the violence in Iraq.
"All these changes, along with the rise of Islamic jihadis (supporters of holy war), are what pushed the worshipper of the cross, the pope, to visit Turkey to revive his so-called dialogue of religions," Al-Qaeda in Iraq said.