US Secretary of State John Kerry has said that he is returning to the nuclear negotiations with Iran with "important gaps" still standing in the way of a deal.

In a press conference in Egypt on Saturday, Kerry said while significant progress had been made in the talks, he cannot say whether the US and other world powers would be able to reach a framework deal with Iran at the end of March.

He spoke from the Egyptian resort of Sharm el-Sheikh, where he attended an economic conference. He is set to travel to Lausanne in Switzerland on Sunday for the resumption of the nuclear talks.

Kerry also repeated the US administration's stand that it wants the "right deal" with Iran with "verifiable set of commitments", and not just any deal.

Kerry said he needed to speak with Iran's foreign minister to assess the effects of a letter from Republicans in the US Congress to Iran's leaders.

Kerry said the letter signed by 47 opposition senators undermined the Obama administration's diplomatic effort, adding that it was "completely without precendent".

At the same time, he also tried to give assurance to Iran and the US negotiating partners Britain, China, France Russia and Germany that the US Congress has "no ability" to change an executive agreement on the nuclear talks.

"The clock is ticking and time is critical," said Kerry. "We continue to remain focussed on reaching the right deal."

The Obama administration has set a March 31 deadline to wrap up the talks, although members of the US Senate had vowed to pass new sanctions against Iran after March 24. 

The P5+1 and Iran hope to reach a formal accord by July 1.

But Iran's supreme leader, Ayatollah Ali Khamenei, has criticised the negotiations, and is to give a closely watched Iranian New Year's address on March 21.

On Thursday Khamenei said the other side in the talks was "deceitful and stabs in the back," according to Iranian news agency ISNA.

Source: Al Jazeera and agencies