* String of attacks kills at least 15 in Bagdad
* Two U.S. troops killed carrying out operations
* Bomber kills 11 troops north of capital
(Recasts with details on US troop deaths)
By Muhanad Mohammed
BAGHDAD, May 22 (Reuters) - A string of bombings killed at least 15 people and wounded 74 more across Baghdad on Sunday and two U.S. troops were killed in central Iraq, officials said.
Violence in Iraq has dropped sharply since the height of sectarian conflict four years ago, but bombings, assassinations and attacks occur daily as the remaining U.S. troops prepare to pull out of the country by the end of this year.
At least 11 Iraqi troops were killed when a bomber blew up his car among a group of soldiers investigating another car bomb outside a checkpoint on a main street in Taji, 20 km (12 miles) north of Baghdad, said the capital's security spokesman, Major-General Qassim al-Moussawi.
"Two vehicles exploded in Taji. The first was a parked car bomb. They were trying to defuse it, when another driver blew himself up," he said.
The two U.S. troops were killed on Sunday during operations in central Iraq, the U.S. military said in a statement with no further details of when or how they died.
Iraqi security forces and police are often targeted by insurgents as Washington prepares to withdraw the last U.S. troops at year-end, more than eight years after the invasion that ousted Saddam Hussein from power.
On Sunday, four roadside bombs and a parked car bomb also targeted an Iraqi Federal Police base in Amil District, southwestern Baghdad, killing two civilians and wounding 15 including three policemen, a security official said.
Two more people were killed and another seven were wounded by another roadside bomb planted near a hospital in Sadr City in northeastern Baghdad.
Iraqi forces will take over full control of security at the end of this year when the remaining 47,000 U.S. troops are scheduled to leave Iraq. U.S. troops are now mainly engaged in training and advising Iraqi forces.
Prime Minister Nuri al-Maliki has said Iraqi forces are ready to contain internal threats. But he has called on Iraq's main political parties to discuss the sensitive issue of whether to extend the presence of some U.S. troops. [ID:nLDE74A17B]
Some officials have suggested a small U.S. force remain to assist Iraqis in areas such as intelligence and border security. But already the idea of U.S. troops remaining has sparked street protests. [ID:nLDE73N0B3]
Three other soldiers were also killed on Sunday by a bomb attached to their Iraqi army vehicle in the town of Tuz Khurmato 170 km (105 miles) north of Baghdad, police said.
Three bombs targeting security forces killed 27 people last week in the northern oil city of Kirkuk, a disputed area with a mixed Arab and Kurd population and a potential flashpoint once U.S. troops leave. [ID:nLDE74I0F4] (Reporting by Muhanad Mohammed, Baghdad newsroom; Writing by Patrick Markey; Editing by Jon Hemming)