French president Francois Hollande has praised the enduring alliance between the United States and his own country and says trust has been restored after a falling out over spying.

Mr Hollande is on a three-day visit to the United States, and has been given the kind of welcome reserved for Washington's close allies.

The relationship has been strained recently over leaked revelations the National Security Agency (NSA) spied on French citizens.

Leaders from many US allies, including Germany's chancellor Angel Merkel, were angered by Edward Snowden's revelation that the US monitored their telephone calls.

But France has been more cautious in its critique, emphasising the importance of its intelligence cooperation with Washington.

"We wanted to fight against terrorism, but we also wanted to meet a number of principles. And we are making headway in this cooperation. Mutual trust has been restored," Mr Hollande said during a joint White House news conference.

However, he did reiterate European concerns about data privacy in the internet age.

"Mutual trust must be based on respect for each other's country, but also based on the protection of private life, of personal data," he said

Last year, information technology contractor Snowden fled from his job at the NSA's Hawaii offices, carrying with him a vast trove of classified intelligence information.

The fugitive turned over the material to journalists, launching what has become a steady drip-feed of revelations about the NSA's activity, much of it embarrassing to Mr Obama's administration.

Outrage in the United States has concentrated on the news that the NSA routinely swept up vast quantities of "metadata" attached to private calls made by ordinary Americans.

But internationally, allied leaders have been angered to learn that their own mobile phone conversations have been tapped by US agents.

Leaders touch on Iran nuclear concerns, Syrian war

In a joint press conference the leaders also expressed a shared commitment to enforcing sanctions against Iran until it is proven its nuclear program is for peaceful purposes.

"President Hollande and I agree on the need to continue enforcing existing sanctions (on Iran), even as we believe that new sanctions during these negotiations would endanger the possibility of a diplomatic solution," Mr Obama said.

"And we remain absolutely united on our ultimate goal, which is preventing Iran from obtaining a nuclear weapon," he said.

Mr Obama and his French counterpart also reiterated that diplomacy is still the best way to resolving the situation in Syria.

Mr Obama expressed frustration with the ongoing crisis, describing the situation on the ground as "horrendous".

"Just as our unity on Syria and the credible threat of force led to a plan for destroying Syria's chemical weapons, we're united on what needs to happen next, there," he said.

"Syria must meet its commitments, and Russia has a responsibility to ensure that Syria complies.

"As talks continue in Geneva, we'll continue to strengthen the moderate opposition, and we call on the international community to stem the flow of foreign fighters into Syria."

Mr Hollande said France also looked to the opposition as a key part of its aim for conflict resolution in Syria.

"Geneva is a possible step in the right direction, but we will have to make headway, we will have to cooperate more, make sure our services cooperate more," he said.

"We need to support the opposition. We need to make sure that the choice is not between dictatorship on the one hand and chaos on the other."

ABC/AFP