Twin car bombs killed 21 people in the Shiite holy city of Karbala in southern Iraq on Monday, while four people died in a suicide attack on a Saudi-funded television channel in Baghdad.

The suicide blasts in Karbala, where religious commemoration ceremonies are due to begin on Wednesday, occurred at around 7:00 pm (1600 GMT) and also wounded at least 47 people, a provincial health official said.

Eyewitnesses said one vehicle exploded at a security checkpoint on the southern outskirts of the city, the site of revered shrines to Imam Hussein and Imam Abbas, and the second blew up a short time later just metres (yards) away.

"Most of the killed and wounded are policemen and civilians," said Salim Kadhim, spokesman for Karbala's health directorate, adding that 21 people had died and 47 were wounded.

"Most of the wounded civilians are pilgrims from other provinces," he said.

The attack came two days before the birthday of the Mahdi, the 12th and last imam. Shiites -- the majority of Iraq's population -- believe the missing imam will return to Earth on the Day of Judgment.

Hundreds of thousands of pilgrims are expected in Karbala and the nearby holy city of Najaf for the ceremonies.

US and Iraqi officials have warned of the dangers of an upsurge in violence as negotiations on forming a new governing coalition have dragged on, more than four months after parliamentary elections.

Meanwhile, a suspected Al-Qaeda suicide bomber blew up a car outside the Baghdad offices of Al-Arabiya television in the city centre, killing four people a month after the Dubai-based channel was warned of insurgent threats.

The bomber struck at around 9:30 am (0630 GMT), leaving a massive crater and sending a plume of smoke into the air that could be seen from several kilometres (miles) away.

Majid Hamid, a journalist for the pan-Arab satellite channel, said four people were killed -- three security guards and a female office assistant. That toll was confirmed by an official at Al-Yarmuk hospital in west Baghdad.

An interior ministry official, speaking on condition of anonymity, put the casualty toll at three dead and 16 wounded. Former deputy prime minister Salam al-Zawbayi was among the wounded.

"There was a huge explosion that shook the building -- all the rooms were damaged," Al-Arabiya journalist Tareq Maher told the channel in a live broadcast.

The street in front of the channel's offices was covered in shards of glass and debris, with nearby buildings and several cars showing signs of damage.

Baghdad security spokesman Major General Qassim Atta blamed Al-Qaeda for the attack.

"These are the methods of Al-Qaeda," he told AFP. "The goal of this operation was to attract media attention."

Atta said the explosives-laden vehicle had passed through a checkpoint, and charged that there may have been "cooperation" between the driver and the guards.

Major General Jihad al-Jaabari, the head of the Iraqi army's explosives handling unit, said the bomber was an Iraqi and added that the vehicle was carrying more than 100 kilograms (220 pounds) of ammonium nitrate.

"He was waved through, but if they had searched him, it would have been easy to find the explosive material," Jaabari said.

Monday's bombing was the latest sign of the threat facing journalists in Iraq, and came just a month after Al-Arabiya closed its Baghdad offices citing government warnings of a threat of insurgent attack.

A total of 141 journalists and media workers, the vast majority Iraqi, have been killed since the US-led invasion of 2003, according to the Committee to Protect Journalists (CPJ).

Al-Arabiya, perceived by extremist groups to be pro-Western, is no stranger to attack by suspected Sunni Arab insurgents and to pressure from Iraq's Shiite-led government.

In October 2006, a car bomb targeting the channel's then bureau killed seven people and wounded 20.

In February 2006, presenter Atwar Bahjat and two colleagues were kidnapped and murdered in the mainly Sunni town of Samarra as they covered the bombing of a revered Shiite shrine, an attack that plunged Iraq into sectarian bloodshed.

The channel's ownership by Saudi and other Gulf investors has also made it the focus of suspicions by Iraq's government that its news agenda reflects the concerns of their Sunni governments.