The United States has warned its citizens they could face arbitrary action by authorities when they visit China, urging them to "exercise increased caution".

The travel advisory update by the State Department on Thursday follows the detentions of several Canadians in China last month amid heightened diplomatic tensions in the wake of the arrest in Canada of a top executive in the Chinese telecoms giant Huawei.

The guidance, which maintained the warning at "Level 2", stopped short of discouraging visits but warned about extra security checks and increased police presence in Xinjiang and Tibet, two minority-dominated regions where Beijing has sought to impose strict control.

While the advisory's language was largely the same as earlier, it warned of "arbitrary enforcement of local laws", sudden prohibitions on exiting the country and harassment of US citizens of Chinese heritage.

Rising tensions

The warning comes after the detentions by Chinese authorities in December of a number of Canadians, including businessperson Michael Spavor and Michael Kovrig, a former diplomat and an adviser with the International Crisis Group think-tank.

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China says both men were suspected of endangering state security. Earlier on Thursday, China's top prosecutor said the two Canadians had "without a doubt" violated the law.

Tensions between the North American countries and China increased after Canadian police in Vancouver arrested Meng Wanzhou, chief financial officer of Huawei, on December 1 at the request of the United States.

US prosecutors have accused her of misleading banks about transactions linked to Iran, putting the lenders at risk of violating Washington's sanctions.

Meng's arrest shook financial markets over concerns it would exacerbate US-China tensions as the countries seek to negotiate their way out of a bitter trade dispute which has seen Washington slap tariffs on $250bn worth of Chinese goods, and Beijing responding in kind.

China's own warning

In its previous travel advisory for China issued in January last year, the State Department urged US citizens to "exercise increased caution" in the country because of "the arbitrary enforcement of local laws and special restrictions on dual US-Chinese nationals".

The latest guidance repeats that warning but also warns about China's use of "exit bans" that would prohibit US citizens from leaving the country, sometimes keeping them in China for years.

Beijing last year appeared to hit back at Washington's travel advice by issuing its own guidance to Chinese coming to the US, warning of the risk of mass shootings and the high cost of healthcare as well as robberies, searches and seizures by customs agents, telecommunications fraud and natural disasters.

"Public security in the United States is not good. Cases of shootings, robberies, and theft are frequent,” China's embassy in Washington said in the alert published in July 2018 to its website.

"Travellers in the United States should be alert to their surroundings and suspicious individuals, and avoid going out alone at night."

Despite rising political friction, the two countries remain among the top sources of visitors to each other, with China by far the largest provider of foreign students to the US.

The US has given Level 2 travel warnings to nearly 60 countries since last January.