ANKARA: Turkey has been holding for several days a cargo plane en route from the United Arab Emirates to Iran after its crew refused to document its load, a customs ministry official told AFP on Wednesday.

"The plane owned by a Turkish company had to make an emergency landing at Istanbul's Sabiha Gokcen airport due to technical reasons," the official said on condition of anonymity.

"The company has to document its cargo at our customs until 1000 GMT today (Wednesday), for security purposes otherwise we'll do what's necessary," he added, without elaborating.

The official said that Turkish authorities would have the right to check the plane's cargo if the company refused to reveal the items on board.

"It is most probably carrying gold," he speculated.

An official from the Iranian embassy in Ankara told AFP that they had not been informed by the Turkish foreign ministry of the plane's grounding.

"Authorities will take necessary steps if there is any irregularity in the conduct of business," another Turkish official said.

But officials declined to comment if the cargo of the plane was documented before the given time and if the plane was allowed to resume its journey.

The incident comes amid indirect gold trade between Turkey and Iran, with the trade to settle Turkish imports of Iranian natural gas reportedly being done via United Arab Emirates to bypass US-led sanctions against Tehran.

Turkey could have faced recession if it was not for the boom in exports, particularly in gold sales to Iran, Economy Minister Zafer Caglayan said in December.

"Turkey ships 60 percent of its gold to Iran and (the) rest to the United Arab Emirates and other countries. But we are not Iran's only market," the minister added.

Turkey sold $6.5 billion worth of gold to Iran and another $4.2 billion worth to the United Arab Emirates in the first 11 months of 2012.

Ankara is under severe pressure from its Western allies to reduce imports of natural gas from Iran owing to Tehran's disputed nuclear programme.

Fearing that Tehran aims to acquire nuclear weapons, the United States has led Western powers to impose ever tougher sanctions against Iran which rejects the charge outright, insisting its programme is for peaceful purposes only.

Under a law approved last year, Washington threatened to penalise foreign financial institutions over transactions with Iran's central bank, which handles sales of the country's key oil and gas exports.

On November 30, the US Senate unanimously approved new economic sanctions aimed at further crippling Iran's energy, shipping and port sectors.

But shortly thereafter, Washington extended exemptions from the sanctions to nine major economic powers, including Turkey, China, Taiwan, India and South Korea.

Ankara in return has cut its Iranian oil purchases by 20 percent. Energy Minister Taner Yildiz said last month Turkey met half of its crude oil needs from Iran in 2011 but was now trying to source more from Libya, Saudi Arabia and Russia.

But Turkey expects the latest round of US sanctions against Tehran will not cover natural gas imports.

Iran is Turkey's second-biggest natural gas supplier after Russia, and third biggest in oil.

Iran's economy is struggling to cope with tightening sanctions imposed by the United States and the European Union over the past two years.

Turkey says it is bound only by UN sanctions against Iran, and Turkish officials insist that Turkey will keep buying natural gas from Iran which supplies up to 20 percent of the gas it consumes.