The planned signing of a key nuclear fuel agreement between Iran and Russia has been delayed due to prolonged negotiations, the Iranian Atomic Energy Organization (IAEO) announced Saturday in Tehran.

Alexander Rumyantsev, head of Russia's Federal Atomic Energy Agency, arrived in Tehran on Friday to sign the deal on the return of the spent fuel of Iran's Bushehr nuclear power plant.

The agreement was previously expected to be signed after Rumyantsev and his Iranian counterpart Gholamreza Aghazadeh held a discussion on Saturday morning.

However, spokesman of the IAEO said at noon Saturday that the deal had not been signed and the news conference originally scheduled to follow the signing had also been postponed.

"The negotiations have been prolonged. A press conference might be held tomorrow," said the spokesman.

Bushehr plant, Iran's first nuclear power plant, is being built with Russia's aid in a Gulf island in the southern province of Bushehr.

The United States, accusing Iran of secretly developing nuclear weapons, has pressed Moscow to abandon the project.

In order to prevent Iran from making nuclear weapons with spent fuel, Russia conditions delivery of nuclear fuel to Iran on an agreement signed between the two sides assuring all spent fuel would be returned to Russia.

Iran at first refused to sign the requested agreement, and then softened its stance.

However, the repeated failures in reaching such an agreement have delayed the operation of the Bushehr nuclear plant. On Aug. 22, 2004, Iran said the plant would become operational in October 2006, a year delayed against the schedule.