(Adds detail about U.S. strike, paragraph 4)

By Andrea Shalal and Susan Heavey

WASHINGTON, March 25 (Reuters) - Islamic State's second in command and other senior leaders were likely killed this week in a major offensive targeting its financial operations, U.S. Defense Secretary Ash Carter said on Friday, the latest setback for the militant group.

Carter told a Pentagon press briefing the United States believes it killed Haji Iman, a senior leader in charge of finances for the self-declared caliphate, and Abu Sarah, who Carter said was charged with paying fighters in northern Iraq.

U.S. Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, told the briefing the deaths reflected "indisputable" new momentum in the fight against Islamic State.

U.S. special forces carried out the strike against Haji Iman, officials told Reuters. The original plan was to capture, not kill, him. But after the commandos' helicopter was fired on from the ground, the decision was made to fire from the air, said one of the officials.

Coalition soldiers rarely operate in Islamic State-held parts of Iraq, where there are no friendly forces to help if a mission runs into trouble.

Dunford said he expected to increase the level of U.S. forces in Iraq from the current 3,800 and bolster the capabilities of Iraqi forces preparing for a major offensive against Islamic State in Mosul, but that those decisions had not been finalized.

"We are systematically eliminating ISIL's cabinet," Carter said, using another acronym for the group.

The strike comes amid growing pressure on Islamic State, which is steadily losing territory in Iraq and Syria to U.S.-backed forces.

While the operational significance of removing Haji Iman from the battlefield is not yet clear, it is the latest in a series of strikes against the group's top leaders, including Abu Omar al-Shishani, described by the Pentagon as the group's "minister of war," and a senior Islamic State chemical weapons operative captured by Iraq-based U.S. commandos and turned over to the Iraqi government.

Carter said the killing of Haji Iman, who also went by Abd ar-Rahman Mustafa al-Qaduli and other aliases and who was imprisoned in the region until 2012, would hamper the group's ability to operate inside and outside of Iraq and Syria. But he conceded that alone was not sufficient to cripple it.

"These leaders have been around for a long time. They are senior, they're experienced, and so eliminating them is an important objective and it achieves an important result," he said. "But they will be replaced and we'll continue to go after their leadership and other aspects of their capability."

MOSUL OPERATION

U.S. officials said they were helping Iraqis prepare for a major operation in Mosul to take back territory from Islamic State, which aims to establish a caliphate in Iraq and Syria.

U.S. Marines were providing artillery fire at the request of Iraq to help support Iraqi forces moving into new positions, they said. They said Iraqi forces were carrying out the assault and that the U.S. military was not digging in for a larger ground combat role.

Carter said Haji Iman had been involved in external affairs for Islamic State and played a role in recruiting foreign fighters, but could not confirm he had anything to do with this week's deadly attacks in Brussels, which killed 31 people.

"It's a big blow to IS," said Bruce Riedel, a former CIA analyst and Middle East expert at the Brookings Institution think tank. "He has been an important figure going back to a decade ago during Abu Musaib Zarqawi's era in creating what became ISIS," he said, using another acronym for the group.

Carter did not explain how the United States had been able to successfully target Islamic State leaders like Haji Iman and al-Shishani.

But the military's recent successes suggest that bounties on the group's leaders are yielding tips from members of Islamic State's top command, said Hisham al-Hashimi, an adviser to the Iraqi government on Islamic State.

The United States offered a reward of up to $7 million for information about Haji Iman, according to the State Department website.

Dunford said the U.S. military had significantly increased intelligence sharing with European militaries in recent months as authorities sought to stem the tide of foreign fighters streaming into Iraq and Syria.

He said fighters from more than 100 countries were now in Syria and Iraq and there were estimates the total exceeded 30,000. Greater cooperation was needed by all those countries to avert attacks like the ones in Brussels, he said.

(Additional reporting by Maher Chmaytelli in Baghdad and Phil Stewart in Washington; Editing by Don Durfee and Peter Cooney)

Our Standards: The Thomson Reuters Trust Principles.