America says tests it has carried out prove that oil on board a Russian tanker seized in the Gulf last week was being smuggled from Iraq in violation of UN sanctions.

United States Defence Secretary William Cohen said samples of the fuel oil from the tanker taken for analysis "do reveal that the oil was from Iraq".

"There also was an Iraqi naval officer aboard the ship so it's pretty clear," he added.

The Volgoneft-147 has now arrived in Oman, and has docked at the port of Mina al-Fahl in the capital Muscat.

Russian diplomats were in port to board the tanker.

Omani officials said the tanker was finally moored 4km (2.5 miles) out at sea.

Legal action

US navy personnel, operating as part of a multinational patrol enforcing UN sanctions on Iraq, boarded the boat on Wednesday.

The Russian Foreign Ministry said on Saturday that the results of its own investigation showed the ship was carrying Iranian heavy fuel oil loaded in Iranian waters.

It demanded the tanker be released as soon as it is unloaded.

But Mr Cohen said it would be up to Oman to decide what to do with the ship once it had been unloaded in Muscat.

He even held out the possibility that the Omani authorities might wish to take legal action against the tanker's crew.

US patrols

The usual procedure if a vessel is found to be carrying smuggled Iraqi oil is to confiscate the cargo, sell it on behalf of the United Nations, and then release the vessel.

Mr Cohen pointed out that the tanker was privately owned, and not the property of the Russian Government.

He also said said the US was stepping monitoring of oil traffic, and had added another warship to patrols in the Gulf since the seizure of the Volgoneft-147.

The US navy-led interception force is policing the Gulf to prevent the smuggling in and out of Iraq of goods banned under UN sanctions imposed after the 1990 invasion of Kuwait.

Iraq is allowed to sell oil worth $5.26bn every six months under UN supervision to buy food and medicine.

Russia insists it is complying in full with the sanctions, but has expressed growing dissatisfaction with a lack of progress in easing or lifting the blockade.

Moscow has warned that relations with Washington could be damaged as a result of the incident.

An official Iraqi newspaper, meanwhile, has accused the US of "piracy".