BAGHDAD, Sept 26 (Reuters) - Bomb attacks killed 57 people and wounded more than 120 across Iraq on Wednesday as suspected al Qaeda militants stepped up a campaign of violence coinciding with the Muslim holy month of Ramadan.

In a mainly Shi'ite district of southwest Baghdad, twin car bombs killed 32 people in one of the biggest attacks to hit the Iraq capital in weeks.

The bombs detonated shortly before dusk, when most people were preparing for the evening meal which ends a daylong fast during Ramadan.

"There has indeed been an increase in violence in the last few days, largely in areas in which al Qaeda in Iraq operates and with al Qaeda in Iraq signatures, as they have sought to ramp up attacks," U.S. military spokesman Major-General Kevin Bergner said.

In northern Iraq a suicide bomber killed 10 people and wounded nine when he struck at the home of a tribal leader opposed to al Qaeda near the town of Sinjar. The bomber's target, Sheikh Kanan al-Juhaimur, was wounded.

At the start of Ramadan two weeks ago Sunni Islamist al Qaeda vowed to escalate attacks and specifically said it would target tribal leaders cooperating with security forces.

"This was an increase that was actually expected some weeks ago given past upturns in violence during Ramadan," Bergner told a news conference, adding the number of attacks was down from levels seen last year and roughly in line with 2005.

U.S. President George W. Bush approved plans earlier this month to withdraw around 20,000 combat troops from Iraq by the middle of next year, saying the move was justified by improved security in Baghdad and the western province of Anbar.

The main Anbar sheikh working with U.S. forces, Abdul Sattar Abu Risha, was killed two weeks ago in a bombing claimed by al Qaeda. The group said others who cooperated with the United States would share Abu Risha's fate.

In the city of Mosul, 390 km (240 miles) north of Baghdad, residents heard four explosions in one hour on Wednesday.

Police said three people were killed and 47 were wounded when a suicide bomber hit a government building under construction. Most victims were builders.

Two car bombs at army checkpoints in the east of the city killed one person and wounded two. Police said they killed another would-be suicide bomber and defused his explosives.

In the town of Shirqat, south of Mosul, two car bombs targeting local police killed seven people and wounded five, police said.

A roadside bomb outside a Sunni mosque in the town of Abi Khasib, five miles (8 km) south of Basra, killed four people, police and a Sunni political party said.

"We are continuing our efforts to suppress (the violence) and keep the pressure on the extremist networks," Bergner said.

WEAPONS FIND

Bergner also displayed explosively formed penetrators (EFPs) which he said were discovered by Iraqi and U.S. soldiers on Tuesday and which he said were being stored by a militant group with links to Iran.

EFPs are a particularly deadly armour-piercing bomb which the United States says Iran is supplying to Shi'ite militias in Iraq to attack American troops -- a charge denied by Tehran.

"This weapons cache was supporting criminals in the Diwaniya area associated with groups that have relied in the past on support from Iranian sources," Bergner said.

Washington, already at odds with Tehran over Iran's nuclear programme, has repeatedly accused Iran of arming Shi'ite militias to attack U.S. troops in Iraq.

Last week American soldiers arrested an Iranian man they accused of smuggling roadside bombs into Iraq and training foreign fighters. The U.S. military said he was a member of the Qods Force of Iran's Revolutionary Guards. Iranian and Iraqi officials said the man was a member of a trade delegation.

Tensions between Iran and the United States were already high over the arrest by U.S. forces of five Iranians in the Kurdish city of Arbil earlier this year.

Alongside the EFPs, Bergner showed what he said were 81 mm Iranian mortar rounds, some made in 2001 and others in 2006. "The only country that makes an 81 mm mortar is Iran," he said. (Additional reporting by David Clarke, Wathiq Ibrahim, Mariam Karouny, Aws Qusay and Haider Salahudeen)