Iran and six world powers have reached provisional agreement on key parts of a deal sharply curtailing Tehran's nuclear program, Western diplomats in talks in Switzerland say.

One of these diplomats said Iran had "more or less" agreed to slash the number of its centrifuge machines by more than two-thirds and to ship abroad most of its stockpile of nuclear material.

As negotiators in Lausanne raced to nail down by midnight Tuesday the outlines of a deal, due to be finalised on June 30, the diplomats cautioned however that things may change.

Iranian diplomats however denied that any tentative agreement on these points has been struck, saying that any reports of specific number of centrifuges and exporting its stockpiles were "journalistic speculation".

"The fact is that we will conserve a substantial number of centrifuges, that no site will be closed, in particular Fordo. These are the basis of the talks," the Iranian diplomat said.

A senior member of the Iranian negotiating team said that the "publication of such information by certain Western media is aimed at creating an atmosphere to disturb the negotiating process".