The U.S. Embassy said no deaths or serious casualties were reported in the Green Zone attacks — the second major barrage this week launched from Shiite areas. Two rockets landed on Prime Minister Nouri al-Maliki's compound, but did not explode, an Iraqi government security official said, speaking on condition of anonymity because he wasn't authorized to disclose the information.

Al-Maliki was in Basra, where he is supervising the operation against the Shiite militia fighters. At least 22 people were killed in the Basra fighting, officials said.

The violence marked a stunning escalation in the confrontation between the Shiite-run government and al-Sadr's forces, who have bitterly complained about the recent arrests of hundreds of backers.

The clashes also threaten to reverse the security gains achieved by U.S. and Iraqi forces — which have been hailed by Washington as key signs that Iraq was headed for better days.

Al-Sadr declared a unilateral cease-fire last August. That move — along with a U.S. troop buildup and a Sunni alliance with the American forces — has contributed to a steep drop in violence over the past several months.

But the truce is now under serious pressure. Al-Sadr's allies have grown increasingly angry over U.S. and Iraqi raids and detentions — demanding the release of followers.

The cleric recently told his followers that the cease-fire remains in effect, but they were free to defend themselves against attacks.

Al-Sadr's headquarters in Najaf also ordered field commanders with his Mahdi Army militia to go on maximum alert and prepare "to strike the occupiers" — a term used to describe U.S. forces — and their Iraqi allies, a militia officer said. He declined to be identified because he wasn't supposed to release the information.

Lawmakers from al-Sadr's movement announced in a Baghdad press conference that a general strike campaign — which began in selected neighborhoods of the capital and included the closure of businesses and schools — was being expanded nationwide.

Stores and schools were closed in several other predominantly Shiite neighborhoods in the capital, and armed Mahdi Army members were seen patrolling the streets in some Shiite neighborhoods of the capital.

In Basra, Iraqi soldiers and police battled Mahdi fighters for control of key neighborhoods in Iraq's second-largest city, 340 miles southeast of Baghdad. The fighting erupted a day after al-Maliki flew there and announced the security crackdown against the militias.

AP Television News video showed smoke from explosions rising over the city and Iraqi soldiers exchanging gunfire with militia members.

Basra accounts for most of Iraq's oil exports, but an oil ministry official, declining to be identified because he wasn't supposed to publicly discuss the sensitive issue, said production and exports had not been affected by the fighting.

Oil futures fell below $100 a barrel on the New York Mercantile Exchange.

Curfews were also imposed in the Shiite cities of Kut, where a large number of Mahdi Army gunmen were seen deploying on the streets, and Nasiriyah.

In Baghdad, suspected Mahdi Army gunmen exchanged gunfire with security guards of the rival Shiite Supreme Islamic Iraqi Council in Sadr City, police said.

The rising tension led many people in Shiite neighborhoods to stay at home rather than venture into contested streets.

Athra Ali, 27, a government employee who lives in the Hurriyah neighborhood, said she decided not to go to work after seeing many shops closed and streets abandoned.

A university lecturer at Baghdad's Mustansiriyah University also said the institution had closed early and sent students home.

Security in Basra had been steadily declining well before the British handed over responsibility for security to the Iraqis on Dec. 16.

Col. Karim al-Zaidi, spokesman for the Iraq military, said security forces were encountering stiff resistance from Mahdi Army gunmen in the city.

The U.S. military said Tuesday that five suspected militants were killed in Basra while attempting to place a roadside bomb. Ten others were injured after being spotted conducting suspicious activity, the statement said.

British troops remained at their base at the airport outside Basra and were not involved in the ground fighting Tuesday, according to the British Ministry of Defense.

But three British jets provided aerial surveillance for the Iraqi forces, said Maj. Tom Holloway, a military spokesman in Basra.

He said the British jets had not dropped any bombs adding: "They haven't yet asked."

U.S. officials have insisted they are not going after Sadrists who respect the cease-fire but are targeting rogue elements, known as "special groups," that the military believes have ties to Iran. Tehran denies allegations it is fueling the violence.

The U.S. military blamed Iranian-backed Shiite militia factions for a spate of rocket attacks that struck the Green Zone and surrounding areas on Monday.

The Sadrists allege that rival militia factions have infiltrated the security forces and are targeting the movement to gain advantage in provincial elections expected this fall.

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Associated Press writers Bushra Juhi, Sinan Salaheddin and Qassim Abdul-Zahra contributed to this report.

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