BAGHDAD, Iraq — Three car bombs detonated nearly simultaneously just north of Baghdad, killing at least 60 people yesterday as an American commander told members of Congress in Washington that only one Iraqi battalion was able to operate independently of U.S. forces.

The report by Army Gen. George Casey, the top U.S. commander in Iraq, surprised the Senate Armed Services Committee, since Casey had told Congress in June that three battalions were combat ready.

Republican and Democratic senators alike questioned whether U.S. plans could succeed to draw down forces as Iraqi troops are trained.

"It is ... discouraging to hear today that there is only one Iraqi battalion that is fully capable," said Sen. Susan Collins, R-Maine. "It doesn't feel like progress."

A battalion is composed of about 300 troops.

After the car-bomb attacks, police relied on U.S. forces to secure Balad, a largely Shiite city about 50 miles north of Baghdad.

Casey was joined before the Senate committee by Secretary of Defense Donald Rumsfeld, Air Force Gen. Richard Myers, the head of the Joint Chiefs of Staff, and Gen. John Abizaid, the head of the U.S. Central Command.

All four insisted that progress was being made in training and equipping Iraqi forces.

"We are fighting with them side by side on a daily basis, improving their capabilities day by day," Casey said. "We are measuring this very carefully, and we're not going forward with this capriciously."

Rumsfeld and his commanders yesterday hedged their forecasts of U.S. involvement in Iraq, leaving it unclear when troops will be able to come home or how long it will take before Iraqi security forces can defend their homeland.

Casey appeared to retreat from his June assertion that a "fairly substantial" reduction in the 149,000 U.S. troops in Iraq could begin next year. Yesterday, he said reductions were a "possibility," depending on conditions after an Oct. 15 referendum in Iraq on a new constitution and December elections for a new national parliament.

Casey also said the U.S. presence in Iraq was fueling the insurgency because of the perception of a U.S. occupation, making a troop reduction critical to the U.S. mission in Iraq.

Two senior Republicans, John McCain of Arizona and Collins, expressed deep concern about whether Iraqi security forces could take over from U.S. forces.

"There's one fundamental problem with it [the strategy], and that is whether the Iraqis are capable of carrying out their own military responsibilities," McCain said. "General Casey, you're taking a very big gamble here. I hope you're correct. I don't see the indicators yet that we are ready to plan or begin troop withdrawals given the overall security situation, and that isn't just my opinion alone."

Training Iraqi security forces to defend their country without American logistics and other help — an effort that began about two years ago — is a central pillar of what President Bush described Wednesday as the "strategy for victory."

Casey gave no specific reasons for the drop in the number of Iraqi units rated as "fully capable" — those that can operate without American backup — and said "things change in the battalions ... there are a lot of variables that are involved here."

Collins noted that this kind of disclosure "contributes to a loss of public confidence."

Officials did not say specifically why two battalions are no longer rated at Level 1 and thus unable to operate on their own. They said readiness ratings can change for numerous reasons, such as if a commander resigns, or if more training is needed. Casey also said that "Iraqi armed forces will not have an independent capability for some time."

The first car bomb in Balad detonated around 6:45 p.m., near Balad's police headquarters, which sits in the middle of downtown. When it detonated, it killed not only officers, but also nearby shoppers, witnesses there said.

The streets were particularly crowded yesterday because many residents were shopping for the Muslim holy month of Ramadan, which begins next week, said Maj. Fareed Saleh, a Tikrit police officer who was visiting Balad when the attacks occurred. Most of those killed in the first attack were civilians, Saleh said.

As police officers — including Balad Police Chief Kadhim Abdul Razzaq — began tending to the victims, a second car bomb detonated near the same spot. Razzaq was seriously injured in that attack, police said. The third car bomb exploded near a Shiite mosque 10 minutes after that.

Sgt. Abbas Ali, of the Balad Police Department, said many families grabbed the bodies of their dead loved ones, making it difficult for officials to tally how many were killed.

Ali said he expects the total to increase. At least 100 were injured, he said.

"It's chaos inside the city," he added.

American forces cordoned the city after the attacks and issued a curfew, Saleh said.

While Balad is Shiite, its outskirts are mainly Sunni and U.S. forces have recently led military offensives in parts of those outlying areas.

Knight Ridder Newspapers reporters Mohammed al Dulaimy and Hassan al Jubouri contributed to this report, which was supplemented with information from The Washington Post and Los Angeles Times.