Wave of Baghdad car bombings kills 17

That crackdown and other larger operations around Baghdad

are an attempt to buy time for Prime Minister Nuri al-Maliki's government to reach a series of political benchmarks set by Washington aimed at promoting national reconciliation.

Washington has become increasingly concerned at the slow progress towards those benchmarks. Senior Iraqi government officials told Reuters "marathon meetings" of five of Iraq's most senior political leaders were due to begin on Friday.

The U.S. and Iranian envoys will also hold a second round of landmark talks in Baghdad on Tuesday to discuss Iraq's unrelenting violence.

Iranian ambassador Hassan Kazemi-Qomi and U.S. ambassador Ryan Crocker met in Baghdad on May 28 in the most high-profile meeting of the two enemies in almost three decades.

Washington accuses Shi'ite Iran of fomenting violence in Iraq by supporting and arming militants. Iran denies the charges and blames the bloodshed on the U.S.-led invasion in 2003 to topple Saddam Hussein.

The worsening chaos has pushed the two countries, which have not had diplomatic ties since shortly after Iran's 1979 revolution, to seek common ground.

(Additional reporting by Mariam Karouny)

(Writing by Paul Tait, Baghdad newsroom; editing by Tim Pearce))