WASHINGTON (REUTERS) - President Barack Obama said on Tuesday it is unclear whether major world powers and Iran will soon be able to reach an agreement on restricting Iran's nuclear programme.

With more talks set for this week, Mr Obama injected a note of caution, telling a Wall Street Journal forum that, "We don't know if we'll be able to close a deal with Iran this week or next week."

Arguing that a proposed agreement should be accepted by skeptical US allies like Israel, Mr Obama said an agreement with Iran now would buy some time to see if the world would be able to say Iran is not building a nuclear weapon.

He said the proposed deal would permit a modest lifting of economic sanctions on Teheran, allowing a small portion of Iranian assets to be unfrozen.