"Europe should not simply expect the United States to shoulder that burden alone," Obama said on Friday in Strasbourg, referring to Afghanistan.
"This is a joint problem that requires a joint effort," he said.
Obama also warned European leaders and students at a meeting that, for geographical reasons, Europe faced a bigger threat than the US from al-Qaeda.
No troop promises
Both Angela Merkel, the German chancellor, and Nicolas Sarkozy, the French president, met Obama before the summit got under way.
But despite warm support, they gave no indication that they will send more soldiers to Afghanistan, where more than 70,000 troops are already stationed.
Speaking at a news conference with Obama on Friday, Sarkozy gave his support to "the new American strategy in Afghanistan" but added there would be "no strengthening of French troops" in the country.
He said France, which has just been admitted back into Nato's military command structure after a 43-year absence, was ready to do more in the field of police training and economic aid.
Rob Reynolds, Al Jazeera's correspondent reporting from Strasbourg, said the US was seeking to play down Nato members' reluctance to contribute more troops.
"The US is [now saying it is] looking for civilian trainers for the security and police forces in Afghanistan [and] experts in areas of infrastructure and economy," he said.
"Having been told in no uncertain terms by most Nato countries that they are not going to get extra troops, [the US is saying] they are not looking for them."
Despite Sarkozy's unwillingness to commit more troops, Obama said he was "grateful" for Washington's relationship with France.
"France has already been a stalwart ally when it comes to Afghanistan," he said.
Hillary Mann Leverett, a former diplomat and Afghanistan director at the US National Security Council, told Al Jazeera that European leaders are unlikely to pledge troops to Afghanistan for a policy they are unsure about.
"[European nations] are happy to put money up for something they can claim credit for, for example, to train police... but to train the Afghan military, you don't get credit for that."
"Obama needs to dial back expectations that the Europeans and Nato is the answer for Afghanistan. He should look more to regional players."
In another move, Sarkozy said he will accept one Guantanamo Bay detainee, an Algerian, to be settled in France.
The acceptance of the detainee marks a possible breakthrough in Obama's attempt to close the prison camp and resettle dozens of prisoners in foreign lands.
Share Afghanistan burden
Merkel, the summit's co-host, also stressed Germany's commitment to shouldering its share of the burden in Afghanistan after talks with Obama in Baden Baden in Germany, but she also refused to send more troops to Afghanistan.
"We have a great responsibility here. We want to carry our share of the responsibility - militarily, in the area of civil reconstruction and in police training," Merkel said.
"[The Obama administration's approach to Afghanistan] complements fully what Germany has in mind, a comprehensive approach,'' she said.
Britain said it would consider dispatching extra forces to help out during Afghanistan's presidential election, which is scheduled for August.
Hillary Clinton, the US secretary of state, said on Friday that Obama will not push Nato members on the numbers of troops they can deploy, saying: "The Nato summit is not a pledging conference."
Obama has already pledged to send 21,000 more troops to the country, and is considering sending 10,000 more.