Col. Karim al-Zaidi, spokesman for the Iraq military, said security forces concentrated heavily in the city's center encountered stiff resistance from Mahdi Army gunmen.

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

Iraqi authorities on March 24 set an indefinite nighttime curfew on the city, and Iraqi Prime Minister Nouri al-Maliki traveled to the volatile area about 340 miles southeast of Baghdad. The move is a sign of growing security concerns in Iraq's oil capital.

Meanwhile, the U.S. military said on March 25 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.

Security in the city had been steadily declining in Basra well before the British handed over responsibility to the Iraqis on Dec. 16. But the fierce street fighting was a steep escalation in tensions as rival Shiite factions battle for control of the area.

Last month a British journalist working for CBS and his Iraqi interpreter were kidnapped from a hotel. The Iraqi was released after radical Shiite cleric Muqtada al-Sadr's office negotiated a deal, but the Briton remains in custody.

Al-Sadr's organization has threatened that tensions will escalate in Basra if members of al-Sadr's Shiite Mahdi Army are targeted.

"We are calling for calm, but this new security plan has the wrong timing," Harith al-Edhari, the director of al-Sadr's office in Basra, said Tuesday. "This plan is a government scheme to target the Sadrists as they did in Diwaniyah and Muthanna."

Al-Sadr's followers also have accused the Shiite-dominated government of exploiting a cease-fire to target the cleric's supporters in advance of provincial elections expected this fall. They have demanded the release of supporters rounded up in recent weeks.

The cleric recently told his followers that although the truce remains in effect, they were free to defend themselves against attacks.

U.S. officials have insisted they are not going after Sadrists who respect the cease-fire but are targeting renegade elements, known as special groups, that the Americans believe have ties to Iran.

On Monday, al-Maliki relieved the top two security officials in Basra, officials said, and spent most of the day meeting with security officials around the city to determine what other security changes were needed.

At least one Iraqi battalion has already been sent to Basra, an official in the defense ministry said, speaking on condition of anonymity because he wasn't supposed to talk to the media. Other battalions may be called from Iraq's southern provinces.

The clashes follow recent fighting elsewhere in the country between U.S. and Iraqi forces and factions of the Mahdi Army. The Mahdi Army has come under severe strains in recent weeks as the U.S. and Iraqi forces detained followers they accuse of belonging to breakaway groups.

The U.S. military has accused Iran of arming and funding Shiite extremists to fight American forces in Iraq. Iran denies the allegation.

Meanwhile, the FBI said it has recovered the remains of two kidnapped U.S. contractors in Iraq. The agency identified the contractors as Ronald Withrow of Roaring Springs, Texas, and John Roy Young of Kansas City, Missouri.

Withrow worked for JPI Worldwide when he was kidnapped near Basra in January 2007. Young worked for Crescent Security Group when he was kidnapped in November 2006 and was kidnapped separately.

The FBI said the investigation into the kidnappings is ongoing.

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

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