The warning came as Iraq's parliament got back to work after a month-long summer break, but it was not immediately clear whether lawmakers would quickly take up key benchmark legislation demanded by Washington.
In a statement posted on an Islamic Web site, the Islamic State of Iraq said the "War Ministry" decided to form special battalions for martyrdom seekers "to pound the bastions of the crusaders and their renegade tails" in Iraq.
"These battalions, with God's help, will perform their duties in an excellent manner during the month of Ramadan and the enemies of God will suffer a lot," the statement said. Last Ramadan, al Qaeda also urged its followers to step up attacks against U.S. and Iraqi forces.
Since the 2003 U.S.-led invasion, the month of Ramadan has seen a spike in violence - especially suicide attacks - in part because some Islamic extremists believe those who die in combat for a holy cause during the period are especially blessed.
The statement said "most of the martyr seekers of these blessed battalions will be from the Ansar (Iraqi) brothers."
The authenticity of the statement could not be verified, but it was published by an Islamic Web forum that usually carries announcements by militant groups.
Al Qaeda in Iraq is blamed for some of the deadliest suicide bombings against Shiite Muslim civilians, as well as numerous attacks on U.S. troops and Iraqi soldiers and police.
The announcement is believed to be aimed at countering what the statement described as "media reports that the mujahideen have been weakened and their attacks were curtailed."
Sunni tribal leaders in some major towns, angry over the movement's attempt to monopolize power and mandate a strict Islamic lifestyle, have turned against al Qaeda and, with U.S. support, have defeated the militants forcing them out of their areas.
Local Sunnis taking up arms against al Qaeda, particularly in Anbar Province, has been cited by President Bush as evidence of military success in Iraq.
Mr. Bush told CBS Evening News anchor Katie Couric Monday during a surprise visit to Iraq that if such success continues, it may be possible to start reducing U.S. troop levels.
Speaking Tuesday to CBS Early Show co-anchor Harry Smith, Sen. Joseph Biden, D-Del., responded to Mr. Bush's hint at possible troop level changes.
"If he's talking about bringing down troops to the pre-surge level, to 130,000, that's not withdrawal. The withdrawal would be getting us out of the middle of that civil war," said Biden, who chairs the Senate Foreign Relations Committee.
In other developments:
Iraq's parliament shrugged off calls in July from Prime Minister Nouri al-Maliki to cancel, or at least shorten, the traditional summer pause saying after putting the break off for a month that there was no point waiting any longer for the premier to deliver the legislation.
The session opened with 158 members of 275 present - enough to form quorum, but the agenda was not immediately announced.
The American commander in Iraq, Gen. David Petraeus, and U.S. Ambassador Ryan Crocker are due in Washington to report to Congress next week on progress in Iraq since the introduction of 30,000 more American troops, including whether advances are being made toward national reconciliation.
While parliament was in recess, al-Maliki attempted to break the impasse with major Shiite, Sunni Arab and Kurdish leaders in a high-level meeting just over a week ago. It brought al-Maliki together fellow Shiite Vice President Adil Abdul-Mahdi, Sunni Arab Vice President Tariq al-Hashemi, the head of the northern autonomous Kurdish region Massoud Barzani and President Jalal Talabani, who is also a Kurd.
They said they agreed in principle on some issues that the U.S. has set as benchmarks for progress, among them holding provincial elections, releasing prisoners held without charge and changing the law preventing many former members of Saddam Hussein's Baath Party from holding government jobs and elected office.
Any agreements reached at the secret meeting will be harder to implement in the diverse 275-member parliament, with the specter of sectarian violence a stark reality just outside the doors.
Mr. Bush told Couric Iraq's parliamentarians are "very clear on the current government structure. What they're having trouble getting to is passing laws. They passed 60 laws last year. They passed a significant budget. They understand we expect them to pass more laws."
However, Sen. Biden told Smith on The Early Show "there is virtually no political progress being made".
"You don't hear any progress about how Sunnis and Shiites, the ones in the midst of the civil war, are beginning to live well with one another.
"Prior to the surge, people were fleeing their neighborhoods at 50,000 people a month. Since the surge, they've been fleeing their neighborhoods at 100,000 per month," Biden said.