SHARM EL-SHEIKH, Egypt (AFP) - US Secretary of State John Kerry said on Saturday that talks on Iran's disputed nuclear programme have made progress, but there were still "important gaps" to overcome.

Kerry, who is attending a three-day international investor conference in Egyptian resort town of Sharm el-Sheikh, said the purpose of these talks was "not just to get any deal, it is to get the right deal".

"We made some progress, but there are still some gaps, important gaps." The Iranian nuclear talks are entering their critical end-game with Kerry due to meet his Iranian counterpart in Switzerland after the Sharm el-Sheikh conference ends.

Six world powers - the US, Russia, China, Britain, France and Germany - aim by the end of this month to nail down the outline of a deal that would put making a nuclear bomb out of Tehran's reach.

"The deadline is approaching. Time is critical," said Kerry. "We continue to remain focused on reaching the right deal."

The parties hope to reach a full 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.

Kerry will travel to the Swiss city of Lausanne Sunday to meet Iranian Foreign Minister Mohammad Javad Zarif.

The US negotiating team, including chief negotiator Under Secretary of State Wendy Sherman and Energy Secretary Ernest Moniz, are scheduled to join him in Lausanne.