By Patricia Zengerle and Steve Holland
WASHINGTON (Reuters) - Vice President Joe Biden led a high-powered delegation to Capitol Hill on Thursday to try to persuade U.S. lawmakers to hold off on any more sanctions against Iran and let delicate diplomatic talks over Tehran's nuclear program unfold.
President Barack Obama is convinced that there is the potential for an international deal to prevent Iran from developing a nuclear weapon but worries that congressional pressure for additional sanctions could complicate negotiations.
Biden, Secretary of State John Kerry and Treasury Secretary Jack Lew held a closed-door session with Senate Democratic leaders and Republican and Democratic members of the Senate Banking Committee to update them on major power talks with Iran. A new round of negotiations is set for next week in Geneva.
An official in Biden's office said the administration's message was that there may come a point when more sanctions are needed, but now may not be the best time for Congress to act.
Congress is weighing increasing sanctions to try to apply more pressure on Iran, which has uranium and plutonium programs that Western officials say are aimed at producing nuclear arms. Iran denies the accusations.
Some prominent lawmakers such as Democratic Senator Robert Menendez, chairman of the Senate Foreign Relations Committee, have expressed scepticism about the need for any delay in imposing new sanctions, saying a fresh round is needed to discourage Tehran's nuclear ambitions.
Administration officials argue that the current sanctions regime has had the desired effect, with new Iranian President Hassan Rouhani, a presumed moderate, seeking a deal to ease them and fulfil a campaign promise to improve the Iranian economy.
Sanctions imposed last year by Washington and the European Union have combined to slash Iran's oil exports by roughly 1 million barrels a day, depriving Tehran of billions of dollars worth of sales and driving up inflation and unemployment.
The official in Biden's office said existing sanctions have "gotten us to where we are today, to have the opportunity to test Iranian intentions to seek an enduring diplomatic solution."
"No one is suggesting an open-ended delay for new sanctions, and there may come a point where additional sanctions are necessary," the official said, but Congress should reserve its ability to legislate when it is most effective.
"The window for negotiation is limited, and if progress isn't made, there may be a time when more sanctions are in fact necessary. We have always said that there would be no agreement overnight and we've been clear that this process is going to take some time," the official said.
(Editing by Warren Strobel and Xavier Briand)