President Barack Obama issued a stern defense of his outreach to Iran on Friday, warning US lawmakers not to derail diplomatic efforts to curtail the Islamic state's nuclear program.

In an end-of-year news conference, Obama said efforts in Congress to pass tougher economic sanctions could damage recent moves to halt Tehran's alleged drive to refine nuclear fuel and build a weapon.

And he once again underlined that any break in the diplomatic momentum towards a deal could force Washington into another military conflict with a Middle East power.

Obama said the six-month interim agreement struck last month between Iran and world powers should stand as a test of Tehran's willingness to come to a deal with the international community.

"It is very important to test whether that's possible, not because it's guaranteed but because the alternative is us having to engage in some kind of conflict to resolve the problem with all kinds of unintended consequences," he said.

"It is my goal to prevent Iran from obtaining a nuclear weapon, but I sure would rather do it diplomatically."

The United States and five more world powers -- Britain, China, France, Germany and Russia -- met Iranian envoys in Geneva last month and agreed a six-month deal to reduce nuclear tensions.

Iran agreed to place its nuclear plants under UN watch, to halt new uranium enrichment and to dilute moderately enriched fuel stockpiles in exchange for a loosening of Western economic sanctions.

Washington's goal is to slow Tehran's enrichment drive while drawing the Islamic regime into talks on a final deal that will end fears, denied by Iran, that it intends to build a nuclear weapon.

Iran insists it never intended to build a bomb, but its main regional foes Israel and Saudi Arabia, traditional US allies with good support among Washington lawmakers, want stronger action.

Some influential members of Congress, from both Obama's Democratic Party and his Republican rivals, are pushing to pass laws that would tighten already strict US sanctions.

Obama's Secretary of State John Kerry has warned that this could torpedo the Geneva deal, and Obama himself took the opportunity Friday to warn against any attempt to thwart the diplomatic track.

"We lose nothing during this negotiation period," he insisted. "There is no need for new sanctions legislation -- not yet."

"Now, if Iran comes back and says, we can't give you assurances that we're not going to weaponize... it's not going to be hard for us to turn the dials back, strengthen sanctions even further," he said.

"I will work with members of Congress to put even more pressure on Iran. But there is no reason to do it right now," he added.

On Thursday, 26 US senators introduced a bill to strengthen Iran sanctions, but it was not clear when it would come for a vote, and the chamber is due to go into recess until the new year.

The White House warned that Obama would in any case veto the bill, meaning that it would not become US law.

Iran is following Washington's disputes closely and last week had already reacted angrily when the US administration blacklisted a dozen international firms for allegedly evading previous sanctions.