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Iraqi forces backed by U.S. air strikes have surrounded ISIS fighters in the northern city of Tikrit in a bid to isolate and choke off the fighters in Saddam Hussein's birthplace, according to senior local officials.

"Iraqi security forces were able to secure and control the road from Tikrit to Baiji,” a Brigadier-General in Iraq’s defense ministry told NBC News late Sunday, referring to the nearby town which is adjacent to a key oil refinery. “These forces are around the city of Baiji now, in order to retake it, but ISIS fighters are fighting hard to defend their position inside the city.”

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He added that government forces, which have been protecting the refinery since June, were preparing for a possible revenge attack by ISIS fighters if they lost control of Baiji, which has become a strategically important city for the extremist group. U.S. war planes have carried out air strikes around Tikrit on Saturday and Sunday, the general said. The action was a key reason for "the success of our forces," he said, adding that the city was now "completely surrounded" by Iraqi forces.

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Sunnis flee in fear of Shia revenge assault on Tikrit http://t.co/A5gUrV1q6o (Getty) pic.twitter.com/91RaufE9Bh - The Times of London (@thetimes) October 20, 2014

- Alastair Jamieson

Reuters contributed to this report.