BAGHDAD (AP) – An explosion leveled a Sunni mosque Saturday in Basra, residents said, in the second retaliatory attack in as many days for the toppling of minarets at a prized Shiite shrine in Samarra.

Iraqi police did not immediately respond to the bombing of the al-Ashrah al-Mubashra mosque, witnesses said, raising fears that the city’s Shiite-dominated security forces were unwilling to stop sectarian attacks on Sunni landmarks.

Secretary of Defense Gates was in Baghdad, where a citywide curfew remained in place to prevent mosque attacks or other retaliatory violence. He was expected to press the Iraqi government to move more quickly toward political reconciliation and other vital reforms that many see as critical to gaining control of violence in the country.

Bombers loaded into pickup trucks pulled up to the al-Ashrah al-Mubashra mosque in Basra’s al-Hakimiya district at dawn, residents in nearby houses said. Minutes after they left, a huge explosion tore through the building, leveling it completely.

It was unclear whether there were any guards present at the time, and why Iraqi security forces did not intervene. Witnesses said they saw no sign of any immediate response from police.

As they were leaving, the insurgents wrote graffiti on the mosque complex’s outer wall with the names of revered Shiite saints, witnesses said. They also hoisted a green Shiite flag over a crumbling part of the mosque complex, they said.

Some nearby houses were damaged in the blast, but no injuries were reported.

Basra is Iraq’s second-largest city, located 340 miles southeast of Baghdad.

On Friday, police said bombers posing as television cameramen destroyed another important Sunni mosque near Basra, the Talha Bin al-Zubair shrine. Afterward, Prime Minister Nouri al-Maliki ordered an indefinite curfew in Basra, which remained in effect Saturday.

The attacks were in apparent retaliation for the suspected al-Qaida bombing of the Shiite Askariya shrine in Samarra three days earlier. Wednesday’s explosions brought down the mosque’s towering minarets and stoked panic that Iraq could fall further into a spiral of sectarian killings.

In February 2006, Sunni militants blew up the same shrine’s glistening golden dome, in an attack whose aftermath has killed tens of thousands of Iraqis.

Meanwhile, radical cleric Muqtada al-Sadr called on Iraqi Shiites Saturday to gather next month at the Askariya shrine to show their commitment to their faith.

In a statement, Mr. al-Sadr said the pilgrimage to the shrine will climax July 7, which falls on the birthday of Fatima al-Zahraa, daughter of Islam’s 7th century Prophet Muhammad and wife of Imam Ali, the founder of the Shiite faith.

“I hope the Sunnis of Iraq will be there waiting for you, paving the road with roses and basil leaves, opening their hearts and their homes to you,” he said. “Let your pilgrimage be one of love, peace, security and unity … go bearing olive branches and wearing shrouds.”

In Iraq’s western Anbar province, the remains of 13 members of an Iraqi taekwondo team kidnapped last year were found near the main highway leading to Jordan, police and hospital officials said. The team had been driving to a training camp there in May 2006 when their convoy was interrupted.

Members of the Anbar Salvation Council, a group of Sunni tribal leaders who have partnered with U.S. and Iraqi officials to fight al-Qaida influence in Anbar, found the 13 bodies Friday west of Ramadi, said Anbar police Col. Rashid Nayef.

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Associated Press Writer Sameer N. Yacoub contributed to this report.