* Thirteen gunmen killed in heavy battles

* Blockade on Sadr City eased

* Sadr rails at United States

(Adds Sadr statement)

By Wisam Mohammed

BAGHDAD, April 12 (Reuters) - U.S. and Iraqi forces killed 13 gunmen in clashes and air strikes overnight in the Baghdad stronghold of cleric Moqtada al-Sadr, who said the United States would remain his enemy until the "last drop of my blood."

Authorities eased a blockade on Saturday in the Sadr City district of eastern Baghdad that had trapped residents in the battle zone slum for two weeks.

Gunfire was audible and some roads remained closed, but cars were allowed in and out of some entrances to the slum, home to 2 million Shi'ites and power base of Sadr and his Mehdi Army militia.

Several hundred people have died in clashes between Sadr's followers and U.S. and Iraqi forces since late March, when Shi'ite Prime Minister Nuri al-Maliki launched a crackdown against the militia in the southern city of Basra.

Sadr ordered his fighters off the streets on March 30, but the showdown has continued in his Baghdad stronghold, turning Sadr City into a key front in the five-year-old war.

The Baghdad and Basra fighting has thrust Iraq back onto centre stage of the U.S. presidential election race.

Residents described the night's clashes as among the worst since Iraqi forces launched an offensive into the area a week ago. A U.S. military statement said at least 13 gunmen were killed in one overnight battle.

Reacting to the upsurge in violence, U.S. Defense Secretary Robert Gates appeared to reach out to Sadr on Friday, saying the cleric would not be treated as an enemy if he played a peaceful role in Iraqi politics.

Sadr responded on Saturday by calling Gates a "terrorist" and accusing the United States of bombarding Iraq's cities.

"You (infidels) will always be an enemy and you will remain so until the last drop of my blood," Sadr said in a statement issued by his office in the holy Shi'ite city of Najaf.

"If you don't withdraw from our land or set a timetable for withdrawal acceptable to the Iraqi people, we will resist in the way we see fit."

U.S. military spokesman Major Mark Cheadle said a U.S. convoy was struck by at least 10 roadside bombs while moving to help Iraqi troop in the west of Sadr City overnight.

U.S. forces fired at least one Hellfire missile from drone aircraft and two rounds from the main battle gun of an M1 tank at fighters who targeted them with roadside bombs, rifles and rocket-propelled grenades, the military statement added.

Police said 17 people were wounded in the overnight fighting in Sadr City. The slum's two hospitals said they received at least 33 wounded.

BOMBS CLEARED FROM ROADS

Cheadle said the decision to lift the blockade was "an indicator of the confidence in the battle-tested Iraqi security forces in that area".

Major-General Qassim Moussawi, the government's Baghdad security spokesman, described the situation as stable. He said some roads were still shut to clear away bombs.

The United States has 160,000 troops in Iraq, 20,000 of whom are due to head home by July, but President George W. Bush has resisted pressure from Democrats to commit to further cuts.

The blockade imposed by U.S. and Iraqi troops on Sadr City has led to skyrocketing food prices and trapped residents in the densely populated slum under nightly bombardment.

Nadeem Qasim, a civil servant in the water department, said the situation would not improve as long as Iraqi army vehicles remained in Sadr City and U.S. planes hovered overhead.

"It means the problems and bombardment may resume," he said.

As fighting raged overnight, loudspeakers on mosques blared out speeches in support of the Mehdi Army.

A Reuters correspondent who spent the night inside Sadr City said U.S. helicopters and jets flew overhead before midnight and several of the aircraft could be seen firing missiles.

The sound of heavy gunfire erupted in several parts of the slum and fighters could be seen on the streets carrying rocket-propelled grenades and machine guns into battle.

A U.S. soldier was killed by an improvised explosive device in northwest Baghdad on Saturday. Nearly two U.S. soldiers per day have died in April, the highest toll since September.

In central Baghdad, about 200 people, including men, women and children, staged an anti-U.S. march on Saturday to demand the withdrawal of U.S. troops. (Additional reporting by Khalid Al-Ansary, Aws Qusay and Peter Graff; Writing by Peter Graff and Noah Barkin; Editing by Jon Boyle)

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