A court in China has sentenced a Canadian man to death on drug trafficking charges after his previous 15-year prison sentence was deemed too lenient, a ruling likely to deepen a diplomatic rift between Ottawa and Beijing.

Robert Lloyd Schellenberg, 36, nodded as the judge in the northeast city of Dalian asked him whether he understood the verdict, following a day-long retrial on Monday in which he declared his innocence.

"The court completely rejects the accused person's explanation and defence because it is completely at odds with the facts," the chief judge said in a courtroom packed with observers - among them Canadian embassy officials, AFP news agency reported.

Canadian Prime Minister Justin Trudeau said in Ottawa that he is concerned that China chose to "arbitrarily" apply the death penalty to a Canadian citizen.

Schellenberg was detained more than four years ago and initially sentenced to 15 years in prison in 2016. But suddenly, last month, an appeals court agreed with prosecutors who said the sentence was too lenient and scheduled Monday's retrial with just four days' notice.

Schellenberg was told in court he has the right to appeal to Liaoning High Court within 10 days of receiving the ruling, the court said in a statement, adding that he was involved in organised international drug crimes.

Schellenberg's lawyer Zhang Dongshuo told Reuters news agency that he will likely appeal the sentence.

Monday's sentence comes against the backdrop of the Chinese government's anger over the arrest in Canada of a top executive from telecom giant Huawei last month, on a US extradition request related to Iran sanctions violations.

Chinese authorities have since detained two Canadian nationals - a former diplomat and a business consultant - on suspicion of endangering national security, a move seen as retaliation over the Huawei executive's arrest.

China denies political link

The Dalian court said Schellenberg, who was detained in December 2014, played a "key part" in an international drug trafficking syndicate.

"Not just content with spreading drugs in one country, the syndicate has spread across borders... it is a harm to human health and also to the stability of countries," the court said.

In his final statement before the sentence was announced, Schellenberg said, "I am not a drug smuggler. I came to China as a tourist."

Analysts and rights groups said retrials are rare in China, especially ones calling for a harsher sentence.

"China is going to face lots of questions about why this particular person, of this particular nationality, had to be retried at this particular time," Human Rights Watch's Washington-based China director Sophie Richardson told Reuters.

Ottawa has said it was following the case "very closely" and has provided Schellenberg with consular assistance.

Beijing has repeatedly denied any diplomatic pressure in the case.

The Chinese foreign ministry said on Friday that critics "can stop recklessly suspecting others of politicising legal issues just because they have done so".

In his defence, Schellenberg said he was a victim of a set-up by an international drug ring.

But Chinese prosecutors said he was the principal suspect in a case involving an international syndicate that planned to send some 222kg of methamphetamine to Australia, hidden in plastic pellets which were concealed in rubber tyres.

Two Chinese men have also received sentences - one to life imprisonment, while another was handed a suspended death sentence.

China has executed other foreigners for drug-related crimes in the past, including a Japanese national in 2014 and a Filipina in 2013.

Beijing considers the number of people executed in China each year to be a state secret. International human rights organisations estimate the figure at around 2,000.