BAGHDAD — As Pentagon officials consider whether more US troops are needed to battle the Islamic State, the top US commander in Iraq said Monday that more American military advisers have been deployed close to the front lines as Iraqi security forces wrestle for control of Mosul.

Army Lieutenant General Stephen Townsend said the advisers, numbering about 450, were ‘‘operating closer and deeper into Iraqi formations’’ as a new assault on western Mosul entered its second day.

US commanders made the adjustment during the fight for the eastern side of the city, which began in October and ended last month, and the deployment has continued with the attempt, beginning Sunday, to capture western Mosul, the last major ISIS stronghold in Iraq.

It marks the first time the US military has acknowledged how close American service members are to the front lines as it assists what Townsend described as a force of more than 40,000 Iraqi police and soldiers fighting to retake Mosul.

The battle for the western half of the Iraqi city is likely to go on for months in urban neighborhoods where up to 3,000 militants are believed to be entrenched, US military officials have said.

Townsend’s comments came during a visit to Baghdad by Defense Secretary James Mattis, a retired Marine general who led combat forces during the Iraq war. Mattis is the first senior member of the Trump administration to visit Iraq.

He said the US-led military coalition will be able to simultaneously prosecute the war against the Islamic State in Mosul and the Syrian city of Raqqa, the capital of the group’s so-called caliphate, along with operations against militants in other cities.

‘‘We’re going to continue to go after them until we destroy them and any kind of belief in the inevitably of their message,’’ Mattis told reporters after a day of meetings with senior US commanders and Iraqi officials, including Prime Minister Haider al-Abadi.

‘‘They are going to be shown exactly what they are, which is a bunch of murderous relics, to put it bluntly,’’ he said.

Mattis is in the middle of a 30-day review of the US strategy to defeat the Islamic State that is expected to make recommendations to the White House on whether additional US troops are needed or whether new authorities should be granted to American forces to help prosecute the campaign.

About 5,000 American service members are now deployed in Iraq.

Iraqi forces advanced Monday into the southern outskirts of Mosul.

Iraqi helicopters fired rockets at the village of Abu Saif early Monday, targeting a hill that overlooks the city’s airport, the Associated Press reported. By noon, the forces entered the village and gained control over much of the strategic hill as fighting was still raging.

Separately, militarized police in armored vehicles were moving toward the sprawling Ghazlani military base on the southwestern outskirts of the city.

A US-led coalition has been providing close air support throughout the campaign to retake Mosul.

US special operations forces are embedded with some Iraqi units and American troops are in Iraq providing logistical and other support.

Discussing the Syrian operation against the Islamic State, the defense secretary said the United States and its allies are still sorting out what the fight for Raqqa will look like, and whether Turkish forces will be involved.

The issue is considered particularly sensitive because the Turks view Kurds allied with the United States as terrorists, while US officials view them as the most credible local fighters.

Reuters reported Sunday that Turkey has submitted two plans to Washington for the Raqqa battle that would rely on local Arab forces potentially backed by the Turkish military, rather than the Kurds.

‘‘The allies are still working it out,’’ Mattis said. ‘‘They’re sharing planning, and that’s all I’m going [to say] right now. But, the planning is still underway, so it has not all been decided upon who is going to do what and where. We’re working together to sort it out.’’

President Trump has repeatedly vowed to eliminate the extremist group but has provided few details about how his approach might differ from that of the Obama administration, which had partnered with Syrian and Iraqi forces to drive ISIS out of several towns and cities.