"We have a common purpose and a common strategy," said Turnbull, as Obama hailed Australia's role in tackling jihadists who control swathes of Syria and Iraq and carried out deadly attacks in Paris.
"Australia is the second-largest contributor to the counter-ISIL coalition," Obama said, using another acronym for the group.
Both men, who met on the sidelines of a regional summit in Manila, are under political pressure at home to do more to fight the Islamic State.
"Let's assume that we were to send 50,000 troops into Syria," he said curtly answering a journalist's question Sunday.
"What happens when there's a terrorist attack generated from Yemen? Do we then send more troops into there? Or Libya, perhaps? Or if there's a terrorist network that's operating anywhere else - in North Africa, or in Southeast Asia?"
Critics say his White House is not doing enough and warn that history will view the conflict in Syria as "Obama's Rwanda" - where the US was accused by some of failing to act in time to stop genocidal bloodletting there.
Obama has said he will continue the current strategy of air strikes, providing arms to "moderate" Syrian Arab and Kurdish groups and a limited number of special forces on the ground.
Turnbull voiced his support for that mission, saying "we will continue shoulder to shoulder with the United States and our allies in the fight against this type of extremist violence, this type of terrorism".
Turnbull is also facing calls at home to step up the fight.
The predecessor he ousted in September, Tony Abbott, has pointedly called for Australia to consider putting boots on the ground.
"As Tony Abbott well knows, Australia does not act unilaterally. We need legal basis under international law to send our forces into other countries - for their own protection and because Australia plays by the rules," she told Sky News.
After their meeting on the margins of an APEC summit, Obama indicated that Turnbull would visit Washington soon.