The Sunni Arab heart of the Iraqi insurgency seems likely to hold its strength the rest of the year, and some of its leaders are now collaborating with Al Qaeda terrorists, the Pentagon said Tuesday.

In a report assessing the situation in Iraq, required quarterly by Congress, the Pentagon painted a mixed picture on a day when the U.S. military command in Baghdad said 1,500 more combat troops have arrived in the country. The extra troops are part of an intensified effort to wrest control of the provincial capital of Ramadi from insurgents.

The report to Congress offered a relatively dim picture of economic progress, with few gains in improving basic services like electricity, and it provided no promises of U.S. troop reductions anytime soon.

On the other hand, it said the Iraqi army is gaining strength and taking lead responsibility for security in more areas.

The U.S. government has struggled for three years to understand the shadowy insurgency in Iraq, which began in the Sunni Triangle west and north of Baghdad. In Tuesday's report, the Pentagon said the "rejectionists" who are a key element of the insurgency are holding their own against U.S. and Iraqi forces.

"MNF-I expects that rejectionist strength will likely remain steady throughout 2006, but that their appeal and motivation for continued violent action will begin to wane in early 2007," the report said. The term MNF-I refers to the Multinational Force-Iraq, the top American military command in Baghdad.

It also said for the first time that the Sunnis who reject the U.S.-based government are collaborating with Al Qaeda.

"Some hardline Sunni rejectionists have joined Al Qaeda in Iraq in recent months, increasing the terrorists' attack options," the report said.

It said a separate element of the insurgency that U.S. officials describe as former loyalists of the Saddam Hussein regime remains an important enabler of the violence in Iraq. But the Saddam loyalists have "mostly splintered" into other groups. As a result, they are now "largely irrelevant" as a threat to the fledgling Iraqi government, said Lt. Gen. Victor E. Renuart, the head of strategic plans and policy for the Joint Chiefs of Staff, who helped prepare the report.

The report also said that while security in much of Iraq has improved, total attacks against U.S. and Iraqi forces have increased in recent months, following the Feb. 22 bombing of the Golden Mosque in Samarra.

President Bush said he remained hopeful that the new Iraqi government will succeed in stabilizing the country.

"Although there's been some very difficult times for the Iraqi people, I'm impressed by the courage of the leadership, impressed by the determination of the people," Bush said Tuesday in the Oval Office during the credentialing ceremony for Samir Sumaidaie, Iraq's ambassador to the United States.

The troop move announced Tuesday involves about 1,500 soldiers from an armored brigade on standby in Kuwait and reflects a deteriorating security situation in the volatile provincial capital of Ramadi. It raises the number of U.S. military brigades in Iraq from 15 to 16 — just five months after the number was cut from 17 to 15. A brigade has at least 3,500 troops.

The administration is under election-year pressure to demonstrate concrete progress in Iraq and to begin reducing U.S. troop levels at a time when the Army and Marines in particular are stretched thin by war deployments.

Anthony Cordesman, an Iraq watcher with the Center for Strategic and International Studies, said Tuesday there is no clear basis for believing U.S. troop levels can be reduced anytime soon without risking further deterioration in the security situation. He said the best measure of progress is not the number of U.S. troops in Iraq but the degree to which their role in counterinsurgency operations is assumed by Iraqis.

"I think, in honesty, that now looks a lot more like 2007 at the earliest (for) really having serious reductions in the U.S. combat role (and) being certain that the U.S. casualty levels are going down on a lasting basis and being able to reduce the costs of the war," Cordesman said in a telephone interview.

Pentagon spokesman Bryan Whitman said there are 130,000 U.S. troops in Iraq. It was not clear whether that included the 1,500 soldiers from two battalions of the 2nd brigade of the 1st Armored Division whose deployment to the Ramadi area was described as "short term" in a U.S. military statement from Baghdad.

A defense official said the two battalions were expected to be in Anbar for a maximum of four months, operating as part of a Marine force. The official was not authorized to discuss such details and so spoke on condition of anonymity.

A third battalion from the brigade in Kuwait was sent to Baghdad in March as part of a broader plan to improve security in the capital during the formation of Prime Minister Nouri al-Maliki's new cabinet. That cabinet was announced and put in place more than a week ago but still lacks ministers of defense and interior, who control the Iraqi army and police. Whitman said that battalion is still operating in the Baghdad area.