The US Congress and President Barack Obama are on a fast track toward confrontation over sanctions on Iran.

A bipartisan group of senators is pushing a new round of penalties despite the president's warning that they would scupper delicate talks underway to prevent Teheran from being able to develop a nuclear weapon.

The Senate Foreign Relations Committee's hearing on Wednesday on Iranian sanctions is the opening round of the new Republican-led Congress' first foreign policy fight with the White House.

Obama came out swinging on Friday, telling Congress he would veto any Iran sanctions bill that lands on his desk. "Hold your fire," Obama told Congress while standing at the White House alongside British Prime Minister David Cameron, who took the unusual step of calling US senators to lobby against a sanctions bill.

Time is running out to reach a deal with Iran, which says its nuclear program is peaceful and exists only to produce energy for civilian use.

Stiff opposition

Talks have been extended until July, with the goal of reaching a framework for a deal by the end of March. Both Obama and Iranian President Hassan Rouhani face stiff opposition to negotiations from conservatives in their respective homelands.

Moreover, a Republican victory in the 2016 presidential election would make renewed talks with Iran unlikely.

Obama said that if there is new sanctions legislation, Iran could walk away and say the "US was operating in bad faith and blew up the deal".

Moreover, he said the willingness of US international partners to enforce existing sanctions against Iran would wane.

"The sanctions that we have in place now would potentially fray because imposing these sanctions is a hardship on a number of countries around the world," Obama said. "They would love to be able to buy Iranian oil."

Republicans worry that the Iranians are just buying time so they can continue enhancing their nuclear programs. Freshman Republican Senator Tom Cotton of Arkansas has gone so far as to call the negotiations a "dangerous farce". Republicans and some Democrats argue that having more sanctions lying in wait would push Iran to make more concessions at the negotiating table.

A bill drafted by Republican Senator Mark Kirk of Illinois and Democratic Senator Bob Menendez of New Jersey would not impose any new sanctions during the remaining timeline for negotiations. A draft of the bill says that if there is no deal by July 6, the sanctions that were eased during negotiations would be reinstated. After that, sanctions would be stepped up every month.

On Aug 3, new sanctions would take effect on Iran's petroleum industry, followed on Sept 7 by new restraints on nations that import Iranian oil. On Oct 5, the US would slap travel and financial sanctions against more Iranian officials. On Nov 2, new sanctions would be levied on foreign banks that do transactions with Iran's central bank.

AP - Xinhua