President Barack Obama has told an audience of US troops their comrades fighting Islamic State (IS) militants in Iraq will not have a combat mission.
Mr Obama said he would not commit "to fighting another ground war in Iraq".
His comments at a military base in Florida came a day after a top US general said he would recommend ground troops if the air strikes failed.
The US has already undertaken 174 air strikes against IS in Iraq since mid-August.
In the most recent air strikes on Tuesday and Wednesday, US forces destroyed two IS armed vehicles north-west of Erbil and several units south-west of Baghdad, according to US Central Command (Centcom).
Mr Obama's new strategy calls for similar attacks in Syria, and calls on a coalition of 40 countries to confront the militant group.
On Wednesday, the US House of Representatives approved his request to help arm and train moderate rebels in Syria to fight IS.
The jihadist group controls large areas of Syria and northern Iraq. It has 20,000-31,000 fighters in Iraq and Syria, according to CIA estimates.
Mr Obama arrived overnight in Tampa, Florida, where Centcom - responsible for the Middle East and Central Asia - is based.
After briefings with top military officials, Mr Obama told an assembly of troops that "the American forces that have been deployed to Iraq do not and will not have a combat mission".
But the US would see that the group was eventually defeated, Mr Obama said - "If you threaten America, you will find no safe haven."
"We cannot do for the Iraqis what they must do for themselves," Mr Obama said.
"After a decade of massive ground deployments it is more effective to use our unique capabilities in support of our partners on the ground so they can secure their own countries' futures," Mr Obama said.
Mr Obama highlighted partner countries like France and the UK, which were already flying reconnaissance flights, and Saudi Arabia, which has agreed to host a US-led training programme for Syrian rebel groups fighting IS.
But the US president added there was "some things only we can do".
"Our armed forces are unparalleled and unique," Mr Obama said. "So when we've got a big problem somewhere around the world, it falls on our shoulders."
During testimony at a Senate hearing on Tuesday, Chairman of the Joint Chiefs of Staff Gen Martin Dempsey said he would recommend a ground troop operation to the president if the international coalition failed to destroy IS.
Analysis: Jonathan Marcus, BBC defence correspondent
A commitment to avoid "boots on the ground" is seen as the acid test of President Obama's commitment to avoid drawing the US into another extended war in Iraq. But the US has already deployed some 1,600 military personnel to Iraq.
They are helping to protect US diplomatic facilities, operate drones, and train and mentor Iraqi forces. So far these mentoring teams are at the headquarters level. But many experts believe that this will eventually have to extend to units on the battlefield.
Senior US commanders already envisage a potential wider role for US forces. For now this is anathema to the White House which is still "selling" its new Iraq policy to its domestic audience.
But over time, a refusal to deploy major US combat units may make more sense than blanket statements about "no boots on the ground."
Meanwhile, Iraqi Prime Minister Haider al-Abadi on Wednesday rejected as "out of the question" the possibility that foreign ground troops would be allowed to fight in his country.
"Not only is it not necessary," he told the Associated Press, "we don't want them. We won't allow them. Full stop."
The night before the Iraqi and US leaders' remarks, IS released what analysts described as a video response to the US moves.
The slickly produced, Hollywood-style trailer for a film entitled Flames of War refers to Mr Obama's insistence that US combat troops will not be returning to fight in Iraq.
In an apparent taunt, it depicts wounded US troops, masked executioners standing over kneeling captives, and declares at the conclusion: "Fighting has just begun."