BAGHDAD - Washington has an agreement with Baghdad on privileges and immunities for the growing number of troops based in Iraq who are helping in the fight against the Islamic State group, the new U.S. ambassador said Thursday.

In an exclusive interview, Stuart Jones said Prime Minister Haider al-Abadi has given assurances that U.S. troops will receive immunity from prosecution. Under Iraq's former Prime Minister Nouri al-Maliki, that issue was a major sticking point, ultimately leading to the decision to withdraw all remaining U.S. troops in late 2011.

"That was a different situation, and those troops would have had a different role," Jones said.

"We have the assurances that we need from the government of Iraq on privileges and immunities," he said. "It's in the basis of our formal written communications between our governments and also based on the strategic framework agreement that is the legal basis of our partnership."

The House was expected to vote on a proposed $5 billion expansion of U.S. military operations against the Islamic State group in Iraq, part of a broader $585 billion defense policy bill for Iraq and Syria. Last month, Obama authorized the deployment of up to 1,500 more American troops to bolster Iraqi forces, which could more than double the total number of U.S. forces to 3,100. That's in addition to the 5,000 people working for the U.S. mission in Iraq.

The U.S.-trained and equipped Iraqi military has struggled to recover from its collapse in June, when the Islamic State group captured the country's second largest city, Mosul, and swept over much of northern and western Iraq. The United States began launching airstrikes in Iraq on Aug. 8, and now heads a coalition backing Iraqi and Kurdish ground forces from the air.

U.S. advisory teams, which were previously based in Baghdad and the Kurdish regional capital Irbil, are now fanning out to other locations in the country, including the highly volatile Anbar province in western Iraq, where U.S. troops fought some of the heaviest battles of the eight-year conflict.

This time the troops are operating far from the front lines. "What we're doing is airstrikes," Jones said. "What we're doing is sharing intelligence. We're doing advise and assist and we're doing training - and that's all we're doing."

Part of the plan to boost Iraqi forces includes training, equipping and paying Sunni tribesmen to join in the fight against the Islamic State group, reminiscent of the Sunni Sahwa, or Awakening movement, which confronted al-Qaeda in Iraq starting in 2006.

The Pentagon plans to buy a range of arms for Iraq's tribesmen, including 5,000 AK-47s, 50 rocket-propelled grenade launchers, 12,000 grenades and 50 82-mm mortars. The arms supply, described in a document that will be sent to Congress for its approval, said the estimated cost to equip an initial Anbar-based force of tribal fighters is $18.5 million, part of a $1.6 billion request to Congress that includes arming and training Iraqi and Kurdish forces.

However, recruiting the tribes has been a challenging process since many of the Sunni tribes involved in the Sahwa campaign felt a breach of trust after the American and Iraqi governments' commitment to the program waned.