US President Barack Obama has urged US senators to hold off from proposing more sanctions against Iran to allow time for world powers to complete a deal on Iran’s nuclear programme.
A White House spokesman warned that if a deal with Tehran was not agreed, Iran would continue enriching uranium.
Envoys from the P5+1 group of nations will begin a new round of negotiations with Iran in Geneva on Wednesday.
Iran’s foreign minister said he believed issues could be resolved.
In a message on the video-sharing site YouTube, Javid Zarif said: “We expect and demand respect for our dignity. For us Iranians, nuclear energy is not about joining a club or threatening others. Nuclear energy is about a leap, a jump toward deciding our own destiny rather than allowing others to decide for us.”
President Obama held two hours of talks with senators at the White House on Tuesday, along with Secretary of State John Kerry and National Security Adviser Susan Rice.
In recent days some US legislators have expressed concern that the White House is moving too fast and should take a harder line with Tehran.
“We have the opportunity to halt the progress of the Iranian programme and roll it back in key respects, while testing whether a comprehensive resolution can be achieved,” the White House said in a statement.
It said that if there was not an initial agreement, Iran would grow its stockpiles of enriched uranium, install new centrifuges and develop a plutonium reactor in the city of Arak.
Press secretary Jay Carney said Mr Obama had told the senators new sanctions would be most effective as a consequence if Iran refused to accept the deal now on the table or agreed and then failed to comply.
The president also rejected reports that Iran would receive at least $40bn (£25bn) in sanctions relief.
Since 2006 the UN Security Council has imposed a series of sanctions on entities and people involved in Iran’s nuclear programme.
Separate US and EU sanctions have targeted Iran’s energy and banking sectors, crippling its oil-based economy.
Speaking later at an event sponsored by The Wall Street Journal, Mr Obama said he didn’t know if an agreement would come this week or next, but he said the bulk of the sanctions regime would remain.
“We are not doing anything around the most powerful sanctions. The oil sanctions, the banking sanctions, the financial services sanctions, those are the ones that have really taken a big chunk out of the Iranian economy,” he said.
“All of those sanctions and the architecture for them don’t go anywhere.”
Three days of high-level talks earlier this month failed to achieve a breakthrough.
Iran says its nuclear programme is for peaceful means, but the US and its allies suspect Tehran is trying to develop a nuclear weapon.
In another sign of warmer relations between Iran and the West, the UK’s David Cameron on Tuesday became the first British prime minister in more than a decade to hold a telephone call with an Iranian president.
A Downing Street spokesman said Mr Cameron and President Hassan Rouhani discussed Iran’s nuclear programme and the situation in Syria.
The two men “agreed to continue efforts to improve the relationship” between the UK and Iran, the spokesman added.
The so-called P5+1 are the five permanent members of the UN Security Council- the US, UK, France, Russia and China – plus Germany.