Canada has warned its citizens of the risk of "arbitrary enforcement" of local laws in China after a Canadian man convicted of drug trafficking was suddenly retried and sentenced to death, and Beijing denied another detained Canadian diplomatic immunity.

The Canadian government on Monday updated its travel advisory for China, telling citizens to exercise a high degree of caution while in the country.

The update noted the "risk of arbitrary enforcement of local laws" and highlighted the severe penalties for drug offences, including death.

It came hours after a court inin China's Liaoning province sentenced Robert Lloyd Schellenberg to be executed for drug smuggling following a day-long retrial in which the 36-year-old Canadian had 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, including Canadian embassy officials.

Schellenberg had the right to appeal to Liaoning High Court within 10 days upon receiving the ruling, according to a statement by the Dalian Intermediate People's Court.

Justin Trudeau, the prime minister of Canada, said in Ottawa that he was concerned that China had chosen to "arbitrarily" apply the death penalty to a Canadian citizen.

The Canadian government has said it is following the case "very closely" and has provided Schellenberg with consular assistance.

Schellenberg was detained in 2014 and sentenced to 15 years in prison two years later. But last month, an appeal by prosecutors claiming the sentence was too lenient was suddenly given the go-ahead. Monday's retrial was scheduled with just four days' notice.

Analysts and rights groups said retrials were 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," Sophie Richardson, the Washington-based China director for Human Rights Watch, told Reuters news agency.

Prosecutors said Schellenberg 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 been tried. One was given a life sentence and the other a suspended death sentence.

But Schellenberg's lawyer, Zhang Dongshuo, said prosecutors had not produced any new evidence to justify a heavier sentence. He said he planned to lodge an appeal.

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.

The death sentence adds to the tension between the two countries following the arrest in Canada last month of a top executive from telecommunications giant Huawei, infuriating Beijing.

Canada detained Meng Wanzhou, the company's chief financial officer, on a US extradition request related to alleged Iran sanctions violations. Meng, which denies wrongdoing, has been released on bail and extradition proceedings are due to start next month.

Chinese authorities later detained two Canadian nationals - a former diplomat and a business consultant - on suspicion of endangering national security, in what has been seen by analysts as retaliation for Meng's arrest.

Agencies