WASHINGTON (Reuters) - An Iraqi-led operation to retake the Islamic State stronghold of Mosul is unlikely to take place this year, a top U.S. intelligence official told Congress on Tuesday.

A fighter of the Islamic State of Iraq and the Levant (ISIL) holds an ISIL flag and a weapon on a street in the city of Mosul, Iraq, in this June 23, 2014 file photo. REUTERS/Stringer/Files

The comments by Marine Corps Lieutenant General Vincent Stewart were more pessimistic than some recent predictions by U.S. and Iraqi officials about the pace of the campaign against the militant group.

“Mosul will be a complex operation. ... I’m not as optimistic that we’ll be able to turn that in the near term, in my view, certainly not this year,” Stewart, director of the Defense Intelligence Agency, told the Senate Armed Services Committee.

“We may be able to begin the campaign, do some isolation operations around Mosul,” he said. “But securing or taking Mosul is an extensive operation and not something I see in the next year or so.”

Iraqi forces, backed by U.S. airstrikes, reclaimed the city of Ramadi from Islamic State in late December.

Mosul, however, is a far larger city with a populace made up of many sects. And even in Ramadi, Iraqi forces are still working to secure that city and its environs.

Top Iraqi officials recently have suggested that Mosul, which fell when Islamic State forces routed the Iraqi army in 2014, would be liberated this year.

Vice President Joe Biden said in late January: “I promise you, after Ramadi, watch what happens now in Raqqa in Syria and what happens in Mosul, by the end of this year.” Raqqa is the capital of Islamic State’s self-styled caliphate.

Stewart said that in addition to securing Ramadi, Iraqi forces must secure the Euphrates River valley between the cities of Hit and Haditha before turning to encircling Mosul.

Other U.S. officials believe an operation against Mosul, while not imminent, is still possible before the end of U.S. President Barack Obama’s term. The key, they said, is for the United States and its allies to train additional Iraqi forces.