Istanbul - Ankara and Tehran were unable to reach an agreement on natural gas Friday as Iranian President Mahmoud Ahmadinejad ended his visit to Turkey, Turkish media reported.

A row about how revenues would be divided up and investment terms are believed to be the sticking points, but the reports said the talks would continue.

"Turkey is the most important country for the delivery of gas to Europe," Ahmadinejad was quoted as saying Friday. He said he hoped an agreement would be reached soon.

Tehran and Ankara signed a memorandum of understanding last year about the delivery of Iranian gas to Europe through Turkey.

Owing to its row with Iran over its nuclear programme, the United States is against the deal and has threatened sanctions.

Ahmadinejad signed agreements on expanding trade during his controversial visit to Turkey.

The two sides also agreed to cooperate more closely on protecting their shared borders, the fight against terrorism and drugs and on improving infrastructure.

Trade between the two countries is expected to double to 20 billion dollars in the next four years.

The year 2009 has been designated the year of Iranian-Turkish culture in both countries.

It was Ahmadinejad's first trip to NATO member Turkey. He was expected to fly back to Iran Friday evening.

The international community suspects Tehran of secretly developing nuclear weapons under the cover of developing atomic energy for civilian purposes, an accusation Iran has vehemently denied.

The United Nations Security Council has imposed sanctions on Iran as the leadership in Tehran has refused to give in to UN demands to stop enriching uranium. (dpa)