Turkey has for several days been holding 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 grounding.
The incident comes amid indirect gold trade between Turkey and Iran, which is importing the Turkish gold in exchange for natural gas in an attempt to get round Western sanctions.
Turkey and Iran are reportedly trading gold via the United Arab Emirates to bypass the United States-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.
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"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.
On November 30, the US Senate unanimously approved new economic sanctions aimed at further crippling Iran's energy, shipping and port sectors a year after the Congress passed tough restrictions against Tehran.
The latest US proposal is expected to sail through the US House of Representatives and be signed into law by President Barack Obama.
Iran's economy is struggling to cope with tightening sanctions imposed by the United States and the European Union over the past two years.
An EU measure which took effect in July halted European purchases of Iranian crude oil, and has since caused Tehran's oil exports to Asian customers to decline by between 10-30 percent.
Iran is Turkey's second-biggest natural gas supplier after Russia.
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.