BEIJING (Reuters) - A Chinese court ordered a retrial of a Canadian citizen on drug smuggling charges on Saturday after prosecutors said his sentence of 15 years was too light - a case that could further test relations between Beijing and Ottawa.

Tensions between the two governments have been high since Canada’s arrest of a high-ranking Chinese executive at the request of the United States this month, followed by China’s detention of two Canadian citizens on suspicion of endangering state security.

Robert Lloyd Schellenberg had lodged an appeal after being handed a 15-year sentence on Nov. 20 in the northeastern city of Dalian, the high court for the province of Liaoning said in a statement, adding that he was to have been deported after serving his sentence.

At the appeal hearing, prosecutors said the sentence was too light and improper, arguing Schellenberg was highly likely to have been part of a international drugs smuggling operation and had played a major role in smuggling the drugs, the statement said.

The court said it accepted this argument and ordered a retrial. It added that Canadian diplomats were in court for the appeal.

In Ottawa, the foreign ministry said in a statement that it has been aware of the case for several years and would continue providing consular assistance to Schellenberg and his family. It gave no more details.

It was not immediately clear who Schellenberg’s lawyer was or when the retrial may take place.

A Dalian government news portal said this week Schellenberg had smuggled “an enormous amount of drugs” into China. Drugs offences are routinely punished severely in China.

A Briton caught smuggling heroin was executed in 2009, prompting a British outcry over what it said was the lack of any mental health assessment.

In one development that could lessen tensions, Ottawa said on Friday that a citizen who was detained in China this month had returned to Canada after being released.

A Canadian government source identified the citizen as teacher Sarah McIver. China’s Foreign Ministry said this month that McIver was undergoing “administrative punishment” for working illegally.

McIver was the third Canadian to be detained by China following the Dec. 1 arrest in Vancouver of Meng Wanzhou, chief financial officer of the Chinese telecommunications giant Huawei Technologies Co Ltd. Canada had said there was no reason to believe that McIver’s detention was linked to the earlier arrests.

Foreign Minister Chrystia Freeland did not mention McIver last week in calling for the release of the other two Canadians detained on suspicion of endangering state security. They have each only been allowed to see Canadian diplomats once since being detained.

China’s Foreign Ministry said in a statement it was aware of reports McIver had been released, and referred further questions to the “relevant authority”.

Neither China nor Canada has drawn a direct connection between Meng and the cases of the two other Canadians.

China has demanded Canada free Meng, who is fighting extradition to the United States, where she would face fraud charges that carry a maximum sentence of 30 years jail for each charge. Meng has said she is innocent.