BAGHDAD -- Defense Secretary Robert M. Gates on Thursday made an unannounced visit to the Iraqi capital, whose residents are still reeling from a series of bombings that killed more than 100 people earlier this week.

Gates flew to Baghdad directly from Kabul, where he had spent three days meeting with commanders and military units to discuss the administration's recently announced plan to flood Afghanistan with 30,000 new troops.

Before departing for Baghdad, Gates also met with Afghan generals and other military officials, telling them, "This is a relationship forged in blood." He said that even though some U.S. troops will begin leaving Afghanistan in July 2011, in accordance with President Obama's revamped war strategy, a "large number" will remain in Afghanistan for "some period beyond that."

In Baghdad, Gates was scheduled to meet with a variety of top officials, including Prime Minister Nouri al-Maliki, but the meeting with Maliki did not take place, as the prime minister was preoccupied with the fallout from the Tuesday bombing. The trip was planned before the latest outbreak of violence, a coordinated attack on government facilities on Tuesday that killed at least 127 people and wounded more than 500.

Pentagon spokesman Geoff Morrell said Gates offered condolences to the victims of the violence and assistance to the Iraqi government, including help in tracking down the planners of the attacks and additional security measures.

Gates, Morell said, told President Jalal Talabani and other members of the presidency council: "There's still work to be done. This fight has to be carried out on a continuing basis."

U.S. defense officials insist the rash of bombings will not deter the United States from its plans to reduce the number of American forces in the country to 50,000 by the end of August 2010 and to end combat operations in 2011. There are currently 115,000 forces in Iraq.

Lt. Gen. Charles Jacoby, the No. 2 U.S. general in Iraq, told reporters traveling with Gates that this week's attacks had exposed some "vulnerabilities" in Iraqi security forces but "it would be tough for any government in any country to prevent these kinds of attacks."

Tuesday's blasts exploded near education facilities, judicial complexes and other targets, triggering recriminations against the Iraqi government and its security forces and fueling fears that elections -- as well as the ongoing withdrawal of U.S. forces -- will usher in a new phase in the battle for control of Iraq.

Although that fight is now unfolding primarily in the political arena, many Iraqis fear that a rise in violence and political instability could again turn politicians into combatants.

On Thursday, al-Qaeda in Iraq's umbrella organization claimed responsibility for Tuesday's attacks, the latest in a campaign of powerful bombings apparently designed to undermine the Shiite-led government, which insurgents deride as a byproduct of the U.S. occupation.

In a statement issued on the Internet and posted outside of mosques, the Islamic State of Iraq, a dogmatic organization run by Sunni Arabs, vowed to continue striking at the heart of the Shiite-led government. "The list of targets will grow," the group said in the statement, in which it described the targets as "strongholds of evil and nests of infidelity."

The bombings have severely crippled the government's infrastructure, and exacerbated political tension in the capital. Lawmakers have accused Maliki and heads of security agencies of being too preoccupied with positioning themselves for the upcoming parliamentary election to effectively run the country.

Tuesday's attacks included a large bombing at the temporary headquarters of the Finance Ministry, which had its original headquarters bombed in August. Like the August attack, the blast on Tuesday resulted in the death and wounding of several employees who played a critical role in budget execution, which has for years bedeviled the Iraqi government.

The prime minister, who was scheduled to appear Thursday before lawmakers, has already replaced the army commander of the Baghdad security command, Gen. Abdoul Kanbar, in response to the bombings. In a televised speech Wednesday, Maliki appealed for unity.

"The enemies have closed their ranks and united despite their differences," he said. "It is imperative that we do the same."

Jacoby praised Kanbar, calling him "a patriot and a fine leader" who "has seen security in Baghdad dramatically improve under his command." But, he added, "this is a call for the Iraqi government, and the Iraqi government feels responsible to the people."

U.S. officials said the bombings indicate that al-Qaeda is being forced to husband resources for a few large-scale attacks in an attempt to stay relevant.

Jacoby noted that since U.S. forces turned over security to Iraqi forces in the cities June 30, there have been major attacks only on Aug. 19, Oct. 25 and now Dec. 8. "It describes the weakness of the network we are fighting," he said. "It takes a long period of time to generate an attack where in the past there were attacks every day."

Special correspondents K.I. Ibrahim and Aziz Alwan contributed to this report.

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