BAGHDAD — U.S. military officials said Saturday that the overall number of American troops in Iraq will be reduced by some 5,000 with the withdrawal of a combat brigade from Diyala province. But the number of soldiers in the volatile province will actually increase.

The U.S. command in Baghdad announced earlier this month that the 3rd Brigade, 1st Cavalry Division had begun heading home to Fort Hood, Texas, and that its area would be taken over by another brigade already operating in Iraq.

Col. David Sutherland, commander of the 3rd Brigade, acknowledged concerns that the withdrawal of U.S. troops could lead to a reversal of a decline in violence but said the transfer of the 4th Stryker Brigade Combat Team, 2nd Infantry Division, will actually result in more troops in the province northeast of Baghdad.

"Although our redeployment is part of the downgrade of the troops across Iraq, their presence allows more boots on the ground in the province," he said.

Rear Adm. Gregory Smith, a U.S. military spokesman in Baghdad, said the increase would be about 2,400 troops due to repositioning, but he stressed that the overall U.S. force in Iraq will be reduced by 5,000.

"What's now being left behind ... is an increase of around 2,400 or so troops available to conduct operations to the commanders now on the ground in Diyala," he said at a joint news conference with Sutherland. "The capability will actually increase in terms of the number of soldiers on the ground in the coming weeks and months."

The comments underscore recent warnings by American commanders that northern Iraq has become more violent than other regions despite an overall decline in the number of attacks nationwide as al-Qaida and other militants move there to avoid coalition operations elsewhere.

Rear Adm. Gregory Smith, a military spokesman, said Saturday that Iranian-backed Shiite militiamen were responsible for the bombing of a pet market, one of the deadliest attacks in the capital in months.

Smith said four captured fighters who confessed to the bombing were part of a cell that splintered from the Mahdi Army, the powerful Shiite militia led by Muqtada al-Sadr, an anti-American cleric. Al-Sadr had called on his fighters over the summer to lay down their arms.

"The group's purpose was to make it appear al-Qaida in Iraq was responsible for the attack," Smith said, referring to the homegrown Sunni insurgent group that U.S. officials believe is led by foreigners. Their "aim was to demonstrate to Baghdadis the need for militia groups to continue providing for their security."

The Friday blast killed at least 15 people and shattered a growing sense of calm that had spread over Baghdad as violence began to decline. Smith said the carnage was caused by a bomb filled with ball bearings that was hidden inside a birdcage.

Meantime, Sutherland, who said the brigade of about 5,000 troops will be home by next month, outlined a success story in the Maryland-sized province since his troops arrived 14 months ago and said an increased capability of Iraqi security forces in some areas of northern Iraq would allow the repositioning of troops.

"Diyala is a very different province now then when we assumed control in November of last year," he said at a news conference, pointing to the rampant violence, lack of essential services and corruption issues that were dominant. "Today there is hope in Diyala."

But he said significant acts of violence have dropped more than 68 percent province-wide since a troop buildup began in April, with 200 reported by Nov. 20 compared with 464 in all of October and 1,051 in May.

He credited a movement of local citizens against extremists as well as the troop buildup.

The withdrawal will reduce the current total of 20 combat brigades to 19, and the overall force is to shrink further to 15 brigades between January and July, officials have said.

The total number of U.S. troops will likely go from 167,000 now to 140,000-145,000 by July, six months before President Bush leaves office and a new commander in chief enters the White House.

"The redeployment without replacement reflects overall improved security within Iraq," Smith said.

As the U.S. troop reductions proceed, it should become clear whether the so-called "surge" strategy that increased the U.S. troop presence in and around Baghdad resulted in any lasting gains against sectarianism. Critics note that the divided government in Baghdad has made few, if any, strides toward political reconciliation that the Americans have said is crucial to stabilizing the country.

The acceleration of the U.S. mission away from direct combat to more of a support role also will put greater pressure on Iraqi security forces to bear more of the load.

Contributing: Amit R. Paley, The Washington Post