President Barack Obama has said the deployment of 1,500 more US troops to Iraq marks a "new phase" against Islamic State militants.
He told CBS TV that the new troops, although non-combat advisers, could help Iraq go on the attack against IS.
A US-led coalition has been helping Iraqi and Kurdish ground forces with hundreds of air strikes since August.
Iraqi forces have now reportedly seized large parts of Baiji - home to Iraq's biggest oil refinery - from IS.
Officials said troops now controlled some 50% of the town, about 200km (130 miles) north of the capital Baghdad.
'Pushing them back'
In his interview with CBS's Face the Nation broadcast, Mr Obama said: "Phase one was getting an Iraqi government that was inclusive and credible - and we now have done that.
"Rather than just try to halt IS's momentum, we're now in a position to start going on some offense," he said.
The additional 1,500 troops nearly double the current contingent and are being sent following a request from the Iraqi government.
The troops are there to assist and advise the country's army, and are not involved in combat operations.
A statement from the Pentagon said the troops would be establishing several sites to train nine Iraqi army and three Kurdish Peshmerga brigades.
Mr Obama said: "The air strikes have been very effective in degrading IS's capabilities and slowing the advance that they were making.
"Now what we need is ground troops, Iraqi ground troops, that can start pushing them back."
When asked whether he would send more soldiers, Mr Obama said: "You know, as commander in chief, I'm never going to say never."
But he added that coalition members would also be joining the mission and it could be that fewer US troops would be needed.
One coalition air strike on Friday targeted a gathering of IS leaders near Mosul in northern Iraq, destroying a convoy of vehicles, a US official said.
But he could not confirm whether IS leader Abu Bakr al-Baghdadi was present at the time.
IS supporters have since denied the jihadist commander was there.
On Saturday, Iraq's defence and interior ministries also reported Baghdadi had been injured.
Defence Minister Khalid al-Obeidi posted on his Facebook page saying: "We have confirmation that Abu Bakr al-Baghdadi was wounded in an allied forces air raid... in the town of Mosul last Friday evening.
"We confirm the death of his deputy, Abu-Muslim al-Turkmani. We pray to God not to help [Baghdadi] recover and to speed up his demise," he said.
However, in conflicting reports an interior ministry intelligence official told Associated Press news agency that Baghdadi was hurt on Saturday in an air strike on a meeting in the town of Qaim in Iraq's western Anbar province, 460km (285 miles) from Mosul.
AP quoted Pentagon officials as saying they had no immediate information of any air strike there.
The BBC is unable to verify the reports.
In Baiji, Iraqi security forces used helicopters to attack IS militants. An Iraqi military official told Reuters news agency government troops had entered Baiji from the south and west, taking control of al-Tamim neighbourhood and the city centre.
The official told the agency progress was slow as improvised explosive devices and snipers had to be cleared
Deputy Governor of Salahuddin province Jasim al-Attia told the BBC the IS militants were stranded between the refinery and army troops arriving in numbers.
IS-led militants took Baiji in June, in one of their earliest victories, but did not capture the refinery.
On Friday, a suicide bomber targeted an Iraqi military convoy in Baiji, killing at least eight people including top police officer Lt Gen Faisal Malik al-Zamel.
The BBC is unable to verify the IS reports. BBC journalists in Baghdad in touch with police and local officials have been told that there was little to be found of the attackers' bodies at the scene.
The British Foreign Office has said it was investigating reports that a UK national has died in the attack.