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NAJAF, Iraq, Sept 15 (Reuters) - The political movement of anti-American Shi'ite cleric Moqtada al-Sadr said on Saturday it had withdrawn from Iraq's ruling Shi'ite Alliance, dealing a further blow to Prime Minister Nuri al-Maliki.

The move leaves Maliki's coalition in a precarious position with around half the seats in the 275-seat parliament, although it could survive with the support of a handful of independent lawmakers.

"The political committee has declared the withdrawal of the Sadr bloc from the (Shi'ite) alliance because there was no visible indication that the demands of Sadr's bloc were being met," the movement said in a statement released at a news conference in the holy Shi'ite city of Najaf.

Sadr's bloc in the past has noted a host of grievances, including Maliki's refusal to set a timetable for the withdrawal of U.S. forces from Iraq.

Maliki can count on the backing of two other Shi'ite Islamist parties and the two main Kurdish parties in parliament.

So far no party has launched any push for a no-confidence vote in his government.

Maliki's 16-month-old government has been paralysed by infighting and came under blistering attack from U.S. lawmakers this week for failing to pass laws seen as crucial to reconcile warring Shi'ite and Sunni Arabs.

In a report ordered by Congress, the White House said on Friday that Iraq had met only half of 18 political and security benchmarks set by the U.S. legislature.

The decision by Sadr's movement to quit the Shi'ite Alliance in parliament was not unexpected after the cleric pulled his six ministers from the cabinet in April.

The main Sunni Arab bloc, the Accordance Front, also withdrew it six ministers from the government in early August, saying Maliki had ignored their demands.

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