BAGHDAD/ERbil - A senior Islamic State member and two of his aides were killed south of the Iraqi city of Mosul on Monday in a helicopter raid by US special forces and Kurdish counter-terrorism forces, the Kurdish regional security council said.

A statement identified the leader as Suleiman Abd Shabib al-Jabouri, also known as Abu Saif, and said he was a member of the militant group’s war council. In a separate operation a day earlier, witnesses and Kurdish security sources said troops from a U.S.-led coalition landed a helicopter north of Mosul and seized at least one Islamic State member from a vehicle.

Moreover, the United States will send more troops to Iraq, potentially putting them closer to the frontlines to advise Iraqi forces in the war against Islamic State militants. US Defense Secretary Ash Carter made the announcement on Monday during a visit to Baghdad during which he met US commanders, Iraqi Prime Minister Haider al-Abadi, and Iraqi Defence Minister Khaled al-Obeidi. About 200 additional troops will be deployed, raising the number of US troops in Iraq to about 4,100, a senior US Defense official said. The Pentagon will also provide up to $415 million to Kurdish peshmerga military units.

Carter did not meet Kurdish leaders in person during his visit, but spoke with the president of the Kurdish region, Massoud Barzani, on the telephone. Monday's announcement is the move in the past several months by the United States to step up its campaign against the hardline group. US special forces are also deployed in Iraq and Syria as part of the campaign. Iraqi forces - trained by the US military and backed by air strikes from a US-led coalition - have since December managed to take back territory from Islamic State, which seized swathes of Iraqi and Syrian territory in 2014.

The new US troops will consist of advisers, trainers, aviation support crew, and security forces. Most of the new military advisers are expected to be army special forces, as is the case with the approximately 100 advisers now in Iraq. The advisers will be allowed to accompany smaller Iraqi units of about 2,500 troops that are closer to the frontlines of battle, whereas now they are limited to larger divisions of about 10,000 troops located further from the battlefield.

That will allow the US military to offer quicker and more nimble advice to Iraqi troops as they try to retake Mosul, the largest Iraqi city still under Islamic State control.

The United States will also deploy an additional long-range rocket artillery unit to support Iraqi ground forces in the battle for Mosul, Carter said. There are two such batteries already in place in Iraq.

A member of Islamic State's war council and two aides were killed in northern Iraq on Monday by US and Kurdish commandos in the second helicopter raid in two days in the area by a US-led coalition, Kurdish security sources said. Meanwhile, Islamic State's income and the population under its control have both fallen by about a third, a US-based analysis firm said, describing the declining revenue as a threat to its long-term rule over its self-proclaimed caliphate. Revenue for the ultra-hardline group, also known as ISIS or ISIL, fell to $56 million a month in March from around $80 million a month in the middle of last year, the analysis company IHS said.

Daily oil output dropped to 21,000 barrels from 33,000 barrels over the same time frame, as production facilities suffered damage from air strikes carried out mainly by a US-led coalition."Islamic State is still a force in the region, but this drop in revenue is a significant figure and will increase the challenge for the group to run its territory in the long term," said IHS senior analyst Ludovico Carlino in a report.

The territory under its control has declined by about 22 percent since mid-2014, while the population of that territory has fallen to around 6 million from 9 million.

"There are fewer people and business activities to tax; the same applies to properties and land to confiscate," said IHS senior analyst Columb Strack.

Around 50 percent of the group's revenue comes from taxation and confiscation, 43 percent from oil and the rest from drug smuggling, sale of electricity and donations, the report said.

The group began allowing people sentenced to corporal punishment to be spared in return for cash payments, an indication of financial difficulty, it said.

It also introduced new taxes on such activities as installing satellite dishes or exiting cities. Fines can be imposed for giving wrong answers about the Holy Quran, it said.