"They are temporary forces," coalition spokesman US Colonel John Dorrian told reporters in Baghdad, adding the long-term authorised level of American troops in Syria would remain at 500.
The announcement came as the State Department said the United States would host a meeting of the 68 members of the coalition fighting IS on March 22.
The American military buildup comes amid calls by President Donald Trump for new plans to accelerate the pace of the war against the Sunni Muslim extremists.
IS jihadists are facing simultaneous offensives in northern Syria by government forces, Turkish-backed rebels, and a US-supported alliance of Kurdish and Arab fighters.
In the latest sign they are feeling the squeeze, their chief Abu Bakr al-Baghdadi was reported to have abandoned Mosul, leaving local commanders behind to fight the Iraqi forces.
"He was in Mosul at some point before the offensive.... He left before we isolated Mosul and Tal Afar," a town to the west, said a US defence official.
"He probably gave broad strategic guidance and has left it to battlefield commanders."
Baghdadi, who declared IS's cross-border "caliphate" at a Mosul mosque in 2014, in an audio message in November urged supporters to make a stand in the city rather than "retreating in shame".
- New neighbourhood recaptured -
Iraq launched the offensive to retake Mosul -- which involves tens of thousands of soldiers, police and allied militia fighters -- in October.
After recapturing its eastern side, the forces set their sights on the city's densely populated west.
In recent days Iraqi forces have retaken a series of neighbourhoods and a museum where IS militants filmed themselves destroying priceless artefacts.
Iraq's Joint Operations Command announced Thursday the elite Counter-Terrorism Service recaptured the Mualemeen neighbourhood in west Mosul.
And forces from the Rapid Response Division, another special forces unit, and the federal police worked to clear roads and buildings in other areas, said the JOC.
The area is located on the edge of Mosul's Old City, a warren of narrow streets and closely spaced houses that could see some of the toughest fighting of the battle.
"Currently there is no order from the operations command to advance toward the Old City. We will advance when this order is issued," said Lieutenant Colonel Abdulamir al-Mohammedawi of the Rapid Response Division.
Hundreds of thousands of civilians are believed to still be trapped under IS rule in Mosul.
Those who did manage to escape the city said the jihadists were growing increasingly desperate.
"We were used as human shields," said Abdulrazzaq Ahmed, a 25-year-old civil servant, who escaped along with hundreds of other civilians to Iraqi police waiting outside the city.
Rayan Mohammed, a frail 18-year-old who was once given 60 lashes for missing prayers, said the jihadists were scrambling in the face of the Iraqi offensive.
"They ran away like chickens," he said.
- Deadly suspected coalition strike -
West Mosul is the most heavily populated area under IS control and one of two major urban centres it still holds, along with Raqa in Syria.
In Syria, a US-backed alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces has been advancing on Raqa.
The United States has been leading a coalition since mid-2014 carrying out air strikes against the jihadists in both Syria and Iraq.
Strikes on an IS-held northern Syrian village thought to have been conducted by the coalition killed at least 23 civilians on Thursday.
Among the dead were at least eight children and six women, said the Syrian Observatory for Human Rights.
The coalition said this month that its raids had unintentionally killed at least 220 civilians since 2014 in both countries.
Elsewhere in Syria, Turkish troops and their rebel allies have pushed south from the Turkish border and driven IS out of the northern town of Al-Bab.
Russian-backed government troops have meanwhile swept eastwards from Syria's second city Aleppo and seized a swathe of countryside from the jihadists.
The Observatory said Thursday that 17 IS fighters from Morocco were killed in intense Russian strikes in the east of Aleppo province, where the group has lost swathes of territory.
The US defence official said IS was now looking beyond the seemingly inevitable losses of Mosul and Raqa.
"They... are still making plans to continue to function as a pseudo-state centred in the Euphrates River valley," the official said.