Obama was in Germany for talks with Chancellor Angela Merkel, and both later met the British, French and Italian leaders to discuss the battle against IS in its self-declared "caliphate" across northern Syria and Iraq.
In a speech in the German city of Hanover, Obama hailed NATO partners' progress so far in pushing back IS, which he called "the most urgent threat to our nations".
"A small number of American special operations forces are already on the ground in Syria and their expertise has been critical as local forces have driven ISIL out of key areas," he said, using an alternative acronym for the militant group.
The US forces will not lead the fight on the ground but provide training and advice to local forces against IS.
"These terrorists will learn the same lessons that others before them have, which is: your hatred is no match for our nations, united in defence of our way of life," said Obama.
Syrian opposition group the High Negotiations Committee said boosting the US military presence to about 300 would be "a good step" and help "rid our country of this scourge".
While most world powers agree that IS -- which has boasted of beheadings and other battlefield atrocities as well as terror attacks in Paris and Brussels -- must be defeated, they have backed different sides in Syria's complex civil war.
Western powers have offered some support to moderate rebels, while Russia has sent troops and fighter jets to back the regime of President Bashar al-Assad.
Aiming to end the bloodshed, all sides eight weeks ago agreed a ceasefire, but it has been frayed by escalating violence around Aleppo, with dozens killed by government air strikes and rebel rockets.
Russian Foreign Minister Sergei Lavrov, meanwhile, put some blame for the lack of progress on the US side.
"We came to an agreement with the Americans for them to use their influence on these 'good opposition' groups and to get them to leave, so that no one would hinder the destruction of terrorist groups," Lavrov said.
"But despite these promises made by the US, nothing has been done in the two months since."
But the diplomat said all parties to the International Syria Support Group which agreed the truce are committed to maintaining it, "and no member believes the cessation (of violence) to be over".
- Ground troops 'a mistake' -
=============================
Syria's conflict, which began in March 2011 with widespread anti-Assad protests, has since spiralled into a multi-front war that has killed 270,000 people.
Obama has come under criticism for his handling of Syria's war, with opponents saying he could have done more to stem the bloodshed.
In an interview with the BBC, Obama said that "it would be a mistake for the United States, or Great Britain, or a combination of Western states to send in ground troops and overthrow the Assad regime."
Pressure on Obama to end the bloodshed is increasing in the United States, which in is the throes of a fiercely fought presidential election race, and from European allies who want to halt the massive influx of refugees.
"As a practical matter, sadly, it is very difficult to see how it would operate short of us essentially being willing to militarily take over a big chunk of that country," Obama said.
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