WASHINGTON, Feb 19 (Reuters) - The special U.S. envoy to Afghanistan and Pakistan said he called Pakistan's President Asif Ali Zardari on Thursday and expressed U.S. concern over a deal with Islamists in the Swat valley region.

Veteran U.S. diplomat Richard Holbrooke said in an interview with CNN that Zardari assured him the pact with Islamists was an "interim arrangement" to stabilize the restive Swat region north of the capital Islamabad.

"He (Zardari) does not disagree that the people who are running Swat now are murderous thugs and militants and they pose a danger not only to Pakistan but to the United States," said Holbrooke, who returned to Washington this week after visiting Afghanistan, Pakistan and India as part of his new role coordinating U.S. policy in the region.

The Pakistani government's pact with Islamists to introduce Islamic law in the northwestern Swat valley has raised concern among NATO countries with troops in neighboring Afghanistan that are struggling to quell the Taliban in the border region.

Western officials fear the pact will only encourage Islamist militancy in the region at a time when U.S. President Barack Obama has ordered an additional 17,000 troops to go to Afghanistan.

"I am concerned, and I know that Secretary (of State Hillary) Clinton is and the president is, that this deal which is portrayed in the press as a truce does not turn into a surrender," said Holbrooke. "President Zardari has assured us this is not the case." (Reporting by Sue Pleming; Editing by John O'Callaghan)

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