BAGHDAD, Iraq - Insurgents have sought to intensify attacks during a Baghdad security crackdown and additional US forces will be sent to areas outside the capital where militant groups are regrouping, the new commander of US forces in Iraq said Thursday.

US soldiers of the 6-9 squadron, 3rd brigade, 1st Cavalry Division, briefly detain Iraqis as they inspect their car in Muqdadiyah, Iraq, 90 kilometers (60 miles) north of Baghdad, Wednesday, March 7, 2007, during a routine patrol. [AP]

 US Gen. David Petraeus said the troop buildups outside Baghdad will focus on Diyala province northeast of Baghdad, a growing hotbed for suspected Sunni extremists fleeing the US-Iraqi security operation in Baghdad.

But Petraeus stressed that military force alone is "not sufficient" to end the violence in Iraq and political talks must eventually include some militant groups now opposing the US-backed government.

"This is critical," Petraeus said in his first news conference since taking over command last month. He noted that such political negotiations "will determine in the long run the success of this effort."

Petraeus listed a series of high-profile attacks since US and Iraqi forces began the security sweep three weeks ago, including a suicide blast at a mostly Shiite university and an assassination attempt against one of Iraq's vice presidents.

The Pentagon has pledged 17,500 combat troops to the capital. Petraeus has said the full contingent should not be in place until early June. He declined to say how many US forces will be deployed to Diyala, which the group al-Qaida in Iraq has made one its main staging grounds.

Military officials believe many insurgents have shifted from Baghdad to Diyala to escape the security operation.

"Car bombs have targeted hundreds of Iraqis," Petraeus said. He also denounced the wave of other attacks, including the "thugs with no soul" who have killed more than 150 Shiite pilgrims in the past three days.

"We share the horror" of witnessing the suicide bombings and shootings against the pilgrims, he said.

Hundreds of thousands of Shiite pilgrims have been streaming by bus, car and foot toward the holy city of Karbala, about 50 miles south of Baghdad, for annual religious rituals that begin Friday.

The Shiite religious rites mark the end of a 40-day mourning period for Imam Hussein, grandson of the Prophet Muhammad. Hussein's death in a 7th-century battle near Karbala cemented the schism between Sunnis and Shiites.

Petraeus said US forces are ready to help provide additional security for the pilgrims if asked by Iraqi authorities.

"It is an enormous task to protect all of them and there is a point at which if someone is willing to blow up himself ... the problem becomes very, very difficult indeed," he said.

But Petraeus added that he saw no role for the powerful Shiite militia known as the Mahdi Army, which had sent out fighters to guard the pilgrimage in the past two years.

He said "extremist elements" in the militia have been engaged in "true excesses" in the past - an apparent reference to suspected gangs carrying out targeted killings against Sunnis.