The UN has warned that trades by the commodity companies Glencore and Trafigura could have breached international sanctions by supplying a company linked to Iran's nuclear programme.

The Guardian reported in April that Glencore had traded $659m (£430m) of goods, including aluminium oxide, with Iran last year.

Glencore, which is run by the multi-billionaire Ivan Glasenberg, has admitted that some of its aluminium oxide ended up in the hands of the Iranian Aluminium Company (Iralco), which has provided aluminium to the Atomic Energy Organisation of Iran (AEOI).

A confidential UN panel of experts report on Iran, seen by Reuters, states: "If confirmed, such transactions may reflect an avenue for procurement of a raw material in a manner that circumvents sanctions."

The report, which was complied by a panel of international experts and distributed to some members of the UN security council but which has not been published, says: "Iran continues to seek items for its prohibited activities abroad by using multiple and increasingly complex procurement methods, including front companies, intermediaries, false documents and new routes."

The panel listed 11 violations of sanctions since June 2012 and said it was investigating at least six possible sanction violations; one of those is thought to be Glencore's dealings.

The report calls on member states to show additional vigilance in order to "identify suspicious transactions".

Kodjo Menan, the Togolese ambassador to the UN and deputy chairman of the security council's committee on sanctions, confirmed to the Guardian that members were concerned about the reports of Glencore and Trafigura's dealings with Iran.

A spokeswoman for the British mission to the UN expressed concern that the report, or elements of it, might never be made publicly available if some members of the security council vetoed its publication.

Trafigura, which hit the headlines for dumping toxic oil waste in the Ivory Coast in 2006, has also admitted trading aluminium oxide (also known as alumina) with Iralco.

Dealing with AEOI has been banned by UN sanctions since 2006. Iralco has been subject to sanctions since December 2012. Both Glencore and Trafigura said their deals with Iralco were completed before sanctions came into force.

Glencore said it ceased transactions with Iralco immediately when it learned of its links to Iran's nuclear programme, and the last trade was in October 2012.

"Glencore complies with applicable laws and regulations, including applicable sanctions," a spokesman said.

Trafigura said: "No deliveries have been made or exports received since new EU sanctions were published in December 2012. Trafigura Group companies are compliant with national and international law where applicable."

The UN experts' report comes days after Gary Fegel, Glencore's head of aluminium, told friends that he planned to leave the company.

Fegel, 39, is the ninth-largest shareholder in Glencore, holding a stake in the company worth $800m. He was named as the world's 22nd youngest billionaire by Forbes magazine this year.

Galsenberg has begun a three-month review of Glencore's aluminium business. A company spokesman said that Fegel's exit from the firm and Glasenberg's review were unrelated to the possible sanctions breach allegations.