Obama made his comments after meeting privately with Russian President Dmitry Medvedev on the sidelines of an economic summit in France. Both men wore stern looks as they talked to reporters after the meeting, and Medvedev said it wasn't even worth trying to reach agreement on all issues.
The United States says its plan to build a missile defense shield in Central and Eastern Europe is aimed at countering emerging threats from countries including Iran and North Korea. Russia maintains that the systems could undermine its nuclear deterrent.
For his part Medvedev said the future of missile defense would be solved by future politicians but that he and Obama can help lay the foundation now.
Obama said he and Medvedev also discussed how to encourage transitions to democracy in the Middle East and North Africa, an issue topping the agenda for the two-day Group of Eight summit of industrialized nations that got under way here Thursday.
On missile defense, Obama and Medvedev agreed it is a sensitive issue and suggested it remains so in their relationship. While they agreed to work on it, they showed no signs of reaching an understanding.
Obama's stern expression as he discussed the matter was in stark contrast to his relaxed and affable demeanor during earlier stops on his four-country Europe tour. Medvedev also appeared cool, and leaned away from Obama as he talked. The two men spoke of a strengthened personal relationship, but their body language did not match their words.
Medvedev has warned that failure to cooperate with Moscow on the shield could spark a new arms race.
Medvedev, according to a translator, said he was "satisfied" by his personal relationship with Obama and that it has helped advanced the one between the countries, too.
"It requires a lot of effort, and it requires continuing in the same vein, full of trust, with relations full of trust, between the two presidents," the Russian president said. "It does not mean that we'll have common views and coinciding views on all the issues. It's impossible, and it's not worth trying."
Medvedev underscored that the two countries will have their own interests but have sought a strategic balance.
Tensions over the missile defense issue eased somewhat when Obama replaced a Bush administration plan for European missile defense, but Russia continues to raise concerns that the system proposed by the Obama administration could be upgraded to counter its arsenal.
After their one-on-one meeting, Obama and Medvedev walked the short distance to the G-8 summit site to meet the host, French President Nicholas Sarkozy. Along the way, Obama stopped to shake hands with onlookers behind metal barricades, many wearing blue plastic ponchos against the overcast skies at this seaside resort.
Obama planned to use the summit that got under way Thursday to work with leading economies on ways to support fledgling democratic transitions in Tunisia and Egypt, while also creating incentives to encourage other countries in the region to pursue greater political freedoms.
The summit comes on the heels of Obama's sweeping address at London's Westminster Hall Wednesday, where he cast the U.S., Britain and other like-minded allies in Europe as the world's "greatest catalysts for global action." He will echo a similar theme in his discussions with G-8 partners on the recent Arab uprisings and argue that the political protesters in the Middle East and North Africa share their democratic values.
Obama will also hold one-on-one meetings with Sarkozy and Japanese Prime Minister Naoto Kan.
Obama's meeting with Kan will be the first between the two leaders since the March earthquake and tsunami in northeast Japan that sparked fears of a nuclear meltdown at the damaged Fukushima plant.
The G-8 comprises the U.S., Britain, France, Germany, Italy, Canada, Russia and Japan. The European Union is also represented.
Interim prime ministers from Tunisia and Egypt, where longtime leaders were pushed out of power earlier this year, will join the summit Friday for a special session aimed at identifying their nations' most critical needs as they move toward elections. Representatives from the World Bank and the International Monetary Fund will give G-8 leaders an assessment of what it will take to modernize and stabilize the Tunisian and Egyptian economies.
Obama has called on the G-8 to do more than just offer aid and assistance; he wants leading industrial nations to focus on boosting long-term investment in the region and increasing trade. U.S. officials have said other Arab countries that embark on democratic transitions could also receive financial help.
In a letter to G-8 leaders Tuesday, Secretary of State Hillary Rodham Clinton and Treasury Secretary Timothy Geithner urged the summit partners to help Egypt swap its debt for investments in job creation.
While the U.S. is holding up Tunisia and Egypt as the most successful models to emerge from the Arab unrest thus far, both face significant obstacles on their paths toward democracy. Tunisia has imposed periodic curfews and detained about 1,400 people in continued protests, and in Egypt, sectarian violence has broken out, with Muslims and Coptic Christians clashing in the streets.
Obama will also face questions at the summit about the NATO-led bombing campaign in Libya. Obama said Wednesday that the operation has no clear end date, though he contended it ultimately would be successful in stopping Moammar Gadhafi's attacks on civilians.
Also likely to be discussed are the U.S. troop drawdown plan in Afghanistan, Obama's renewed push for Middle East peace and the continued steps the world's leading economies are taking to recover from the global downturn.
After the summit wraps up Friday afternoon, Obama was to travel to Poland, the last stop on a four-country, six-day tour of Europe that began Monday in Ireland.