The United States will send about 600 more troops to train and assist Iraqi security forces ahead of an anticipated offensive to wrest control of Mosul from "IS," US Defense Secretary Ashton Carter said Wednesday.

"These (US) forces will be primarily to enable Iraqi security forces, and also peshmerga, in the operations to isolate and collapse ISIL's control over Mosul," Carter told reporters, referring to IS by another acronym. Peshmerga are Kurdish fighters.

The reinforcements are the third boost in US troop numbers since April. The increase would bring the total number of US troops in Iraq to nearly 5,300.

Iraqi Prime Minister Haider al-Abadi said in a statement the request for "a final increase in the number of trainers and advisers" came from the Iraqi government.

The additional troops will be in a train and advising role, but "also to protect and expand Iraqi security forces' gains elsewhere in Iraq," Carter said.

The US troops will also be involved in intelligence gathering and providing logistical support. US officials have repeatedly stressed American troops are not in a ground combat role, but special forces have fought with peshmerga on a number of occasions that have been made public. Three US soldiers have been killed in clashes.

"American forces combating ISIL in Iraq are in harm's way... no one should be in any doubt about that," Carter said.

The Iraqi army, allied Shiite militias and peshmerga have retaken broad swaths of territory from IS since it overran Mosul and other parts of the country in June 2014, triggering an US-led coalition air campaign against the group in Iraq and Syria.

In July, Iraqi security forces retook Qayyarah airbase, located 40 miles (65 kilometers) south of Mosul, a strategic logistical and jumping off point for an offensive to retake Iraq's second largest city that could come before the end of the year. The new US forces will be deployed to Qayyarah and the joint Iraqi-US Al Asad air base.

Iraqi forces will take the lead in any ground offensive to retake Mosul, US officials say, but it is likely to be a tough battle as IS has had two years to bolster defenses in a dense urban environment.

Carter said the campaign against Mosul would start "in the coming weeks," but also said that it was up to when Iraqi forces felt they were ready.

US President Barack Obama said last week that the battle for Mosul "is going to be hard, this is going to be challenging," although he expressed confidence Iraqi forces could secure the city quickly.

The United Nations estimates up to a million people could be displaced from Mosul, testing an already overburdened humanitarian situation in Iraq.

The offensive may also heighten tensions between a complicated web of alliances and rivalries between Baghdad, Iran-backed Shiite militias, the Kurds and Sunni Arabs that make up the majority of Mosul's population.

Mosul's Sunni population is wary of Iran-backed Shiite militias, who have reportedly committed human rights abuses in the prosecution of their fight against IS. The Kurds and Iran-backed Shiite militia are also at odds.

The Kurds, who are now about 20 kilometers from Mosul, have said they would not participate in an operation into a city where they are likely to be viewed with suspicion. Their offensive to the east of Mosul has been concentrating pushing IS from towns and villages.

In fighting IS, the Kurds have significantly expanded their territory south of the official borders of their autonomous region in northern Iraq. Their territorial ambitions combined with long-running disputes with Baghdad over oil revenue sharing are likely to create tension between between the two going forward.

Even when Mosul is liberated from IS, the group is likely to shift tactics into traditional insurgency tactics and bombings. It will still also have a strong presence across the border in Syria, where the US is conducting airstrikes against the group and has about 250 special forces embedded with Syrian Kurdish YPG militia.

cw/kms (AFP, Reuters)