(Adds comments from President Bush in paragraphs 10-12)

By Paul Tait

BAGHDAD, June 14 (Reuters) - Three Sunni Muslim mosques were torched and mortar bombs hit Baghdad's heavily fortified Green Zone on Thursday, police said, despite curfews imposed after suspected al Qaeda militants hit a revered Shi'ite shrine.

Thousands of Iraqi and U.S. soldiers were on the streets of Baghdad and other cities trying to enforce the curfews that were imposed after Wednesday's bombing, blamed on al Qaeda, felled the two golden minarets of Samarra's al-Askari mosque.

Police said more than a dozen mortar rounds hit the Green Zone, home to Iraq's parliament and the U.S. embassy, causing some casualties, but they had no further details. Smoke could be seen billowing into the sky from several areas.

A resident at the Rashid Hotel in the Green Zone said one mortar round fell in the hotel courtyard, killing one employee and wounding several. The hotel is home to some members of parliament, journalists and foreign contractors.

The mortars fell while U.S. Deputy Secretary of State John Negroponte was in a nearby building discussing the Iraq government's efforts to meet key political targets set by Washington and aimed at promoting national reconciliation.

"I ... stressed the urgency of making political progress that will reinforce efforts by coalition and Iraqi security forces to restore stability," Negroponte told reporters.

Separately, an Iraqi al Qaeda-led group said it had killed 14 army and police personnel after a deadline expired. A video posted on the Internet showed a masked man shooting the kneeling, handcuffed men in the head.

An attack on Samarra's al-Askari mosque in February 2006 unleashed waves of sectarian violence in which tens of thousands of people were killed, tipping Iraq close to all-out civil war between majority Shi'ite Muslims and minority Sunni Arabs.

The latest Samarra attack, condemned as barbarous by U.S. President George W. Bush, immediately raised fears of similar retaliatory violence, but there has so far not been a repeat of the widespread killings of 2006.

Bush urged Iraqis on Thursday to "reject this provocation."

Addressing a business group in Washington, Bush reiterated the U.S. view that the Samarra bombing "had all the hallmarks of al Qaeda."

"These killers hope that their attacks like this one will create enough confusion and chaos that we will abandon this young democracy," Bush said. "I call on Iraqis to reject this provocation."

PROTESTS

The streets of Samarra were quiet on Thursday, but there was a grim mood as Iraqi soldiers fanned out around the mainly Sunni city. Residents said military snipers could be seen on rooftops.

Police said unidentified gunmen on Thursday attacked the al-Mustafa and Huteen mosques in Iskandariya, south of Baghdad, where the Sunni Grand Mosque was destroyed on Wednesday. The al-Bashir mosque in nearby Mahaweel was also attacked.

Despite the curfews, thousands of supporters of fiery Shi'ite cleric Moqtada al-Sadr took to the streets of Baghdad's sprawling Sadr City slum in noisy but peaceful protests. Protests were also staged in southern Najaf and Basra.

"Out, out occupiers," the Sadr City protesters chanted, in reference to U.S. forces. Sadr has repeatedly called on the government to set a timetable for a U.S. troop withdrawal.

Elsewhere in Baghdad, frustration grew over the seemingly endless cycle of sectarian violence.

"Iraqis are killing every day without any reason, and humans are more precious than any building," said Ammar Salim, a graduate student from the mainly Shi'ite Kadhimiya district.

U.S. commanders have said all American troop reinforcements will be in place by Friday as part of a major security crackdown in the capital. Some 28,000 extra U.S. soldiers are being sent, most of them to Baghdad.

The crackdown was launched in mid-February and had early success in reducing the number of targeted sectarian murders between Shi'ites and Sunni Arabs, but civilian deaths rose by 29 percent in May to almost 2,000.

U.S. military deaths also rose in May to their highest monthly level in two-and-a-half years. (Additional reporting by Waleed Ibrahim and Wissam Mohammed in Baghdad, Matt Spetalnick in Washington)

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