Russia and Azerbaijan have postponed the signing of an agreement to divide up the oil-rich Caspian Sea because crucial documentation is not yet ready.

Aliyev: Due to hold talks with Russian President Vladimir Putin

Azeri President Heydar Aliyev said there were no unresolved issues, but that a few "little nuances" needed to be agreed on.

Mr Aliyev is due to hold two days of talks with Russian Prime Minister Vladimir Putin, but it is not clear if the deal will be signed during such talks.

Russian Foreign Minister Igor Ivanov was also vague, indicating only that there were no conflicts on either side and stressing there was "common understanding" between the two parties.

Long-running dispute

The deal clarifies the two countries' joint border with the Caspian Sea and is seen as a crucial step towards resolving the long-running dispute over who owns which part of the sea.

Three other countries - Iran, Turkmenistan and Kazakhstan - also lay claim to the Caspian Sea, whose status has been debated since the collapse of the Soviet Union in 1991 led to a scramble to take advantage of the landlocked sea's fuel supplies.

Prior to the collapse the sea was governed by an agreement between the then-Soviet Union and Iran.

Compromise

At present boundaries are drawn according to the countries' respective coastlines.

However Iran and Turkmenistan, which have the shortest coastlines out of the five, want them to be changed.

Talks held in Turkmenistan in April this year failed to resolve the dispute, mainly because each country was keen to defend its national interests.

The deal between Russia and Azerbaijan is widely expected to isolate Iran and Turkmenistan and force them to compromise.

The Caspian Sea is thought to hold about a third of the world's oil and gas reserves.