US Secretary of State John Kerry left Thursday for a key economic conference in Egypt, before a new round of talks in Switzerland on Iran's suspect nuclear program.

Kerry is due to arrive Friday in the Egyptian Red Sea resort of Sharm al-Sheikh to attend an international economic conference, set to burnish President Abdel Fattah al-Sisi's global credentials.

Egypt hopes the foreign investor conference will jump-start its battered economy while showcasing international support for Sisi as he battles radical Islamist opponents.

Kerry is due to meet with Sisi -- a former general who led the army's ouster of elected Islamist president Mohamed Morsi in 2013 and then resigned from the military to stand in the 2014 elections. Kerry could also meet with other Arab and European leaders.

Angered by stalled Egyptian democratic reforms, Washington has frozen a chunk of its $1.5 billion in mostly military annual aid to Cairo since October 2013, insisting greater progress must be made.

Apache helicopters, however, have been delivered to the Egyptian military -- a key ally in the fight against Al-Qaeda and Islamic militants sheltering in the Sinai peninsula.

From the divers' paradise of Sharm al-Sheikh, Kerry will fly to Lausanne in landlocked Switzerland on Sunday for fresh negotiations with his Iranian counterpart Mohammad Javad Zarif.

Talks with global powers for a deal reining in Iran's suspect uranium enrichment program in return for sanctions relief are reaching a critical stage as a March 31 deadline for a political framework accord looms.

With uncertainty still surrounding the deal, the State Department has not said how long Kerry will stay in Lausanne, nor when he is expected to return to Washington.

Zarif is also due to meet with EU partners Britain, France and Germany in Brussels on Monday.

"There's no deal yet," State Department spokeswoman Jen Psaki insisted on CNN Thursday.

"The primary objective of any deal is to prevent Iran from acquiring a nuclear weapon," she added.

- Political stunt -

But the negotiations fueled political tensions in Washington earlier this week when 47 Republican senators wrote to Iran warning that Congress could modify any deal struck with US President Barack Obama's administration.

Kerry hit out at that suggestion, saying it was "flat wrong," and German Foreign Minister Frank-Walter Steinmeier on a visit to Washington also denounced the political stunt which he feared could undermine the talks.

"This is not just an issue of American domestic politics, but it affects the negotiations we are holding in Geneva," Steinmeier said before meeting with lawmakers on Capitol Hill.

"Obviously, mistrust is growing on... the Iranian side if we are really serious with the negotiations."

Amid the fallout from the open letter, Iran's Supreme Leader Ayatollah Ali Khamenei also warned the country's top clerical body Thursday against "deceitful" world powers.

President Hassan Rouhani "has selected a nuclear (negotiating) team who are truly good, trustworthy and hardworking," he said, quoted by ISNA news agency, whereas "the other party is deceitful and stabs in the back."