BAGHDAD, June 16 (Reuters) - U.S. and Iraqi forces captured two top militants overnight including one from a gang that may have been involved in the abduction of five Britons in Baghdad in May, the U.S. military commander in Iraq said on Saturday.

General David Petraeus named the two militants as Al Hilfi and Abu Tiba. He said Hilfi was the leader of a secret cell of "extremist elements" within the Mehdi Army militia of anti-American Shi'ite cleric Moqtada al-Sadr.

He told a news conference with visiting U.S. Defense Secretary Robert Gates that Abu Tiba was a member of the Ajur al Dulaimi gang, which "may be associated with some of the British citizens who were kidnapped recently".

The Britons -- a computer expert and his four bodyguards -- were snatched late last month from inside a Finance Ministry building by dozens of gunmen wearing police uniforms.

Iraq's foreign minister, Hoshiyar Zebari, has said he suspected the Mehdi Army was behind the kidnapping because the ministry was close to their stronghold of Sadr City in Baghdad.

Petraeus, who did not give the full names of the militants, did not say if Abu Tiba was part of the Mehdi Army, or whether the gang he mentioned had links to it.

Speaking about Hilfi, he said: "Al Hilfi is the head of the secret cells in the Jaish al Mahdi (Mehdi Army) for all of Baghdad, so that's a very significant capture."

The U.S. military said in a separate statement that one militant was killed and 10 detained in the operations, which Sadr City residents said took place early on Saturday. Police said six people were killed and 20 wounded.

Sadr has made no public comment on the abductions of the Britons, but aides say the operation was beyond the capability of the Mehdi Army.

The U.S. military has said Sadr, who appeared in public for the first time in months in late May, returned from Iran to rein in his militia, which they say is fragmenting into rogue groups.

The country's police are also known to be heavily infiltrated by Shi'ite militias and have often been accused of carrying out kidnappings and killings.

U.S. and Iraqi forces have raided Sadr City in their hunt for the Britons.

They have also carried out numerous operations in the sprawling slum in recent months, detaining militants suspected of bringing sophisticated roadside bombs into Iraq from Iran.

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