Image copyright AFP Image caption Canadians Michael Spavor (L) and Michael Kovrig are being detained in China

The US state department has urged Americans to "exercise increased caution" when travelling to China after a spate of high-profile detentions.

Its updated advice says dual US-Chinese nationals are at particular risk from so-called exit bans that prevent them from leaving.

Canada also revealed that 13 Canadians had been detained since 1 December.

On that day, a top Chinese executive was arrested in Canada at the request of US prosecutors.

Huawei chief financial officer Meng Wanzhou was detained in Vancouver and faces extradition to the US to face fraud charges, which she denies, that are linked to allegations of avoiding US sanctions on Iran.

China has dismissed the US travel warning as unjustified.

"To be frank, the issuance of such a travel advisory by the US side does not hold water," foreign ministry spokesman Lu Kang said.

"From January to November 2018, 2.3 million visits to China were made by Americans, which means 70 per 10,000 American people made the trip, a ratio far higher than that of the Chinese visiting the US.

"So, this figure is a testament to China's safety."

What is the latest US travel advice?

The new advisory warns that dual US-Chinese nationals are at particular risk from so-called exit bans, which it says can be used to prohibit US citizens from leaving China - in some cases keeping them in the country for years.

The exit bans are also being used to try to lure other individuals back to China, it warns.

It advises citizens travelling to China to use their US passport with a valid China visa. They should ask officials to notify the US embassy immediately if they are detained or arrested.

The state department says that as dual-citizenship is not recognised under Chinese law, "US-Chinese citizens and US citizens of Chinese heritage may be subject to additional scrutiny and harassment".

The advisory says exit bans are being used to "compel US citizens to participate in Chinese government investigations" and "to aid Chinese authorities in resolving civil disputes in favour of Chinese parties".

What do we know of the recent detentions?

Three US citizens were accused of committing "economic crimes" and barred from leaving China in November.

Victor and Cynthia Liu, who are the children of a fugitive businessman, and their mother, Sandra Han, have reportedly been detained since June.

The businessman, Liu Changming, is wanted in a $1.4bn (£1bn) fraud case in China and the family has said their detention is an attempt to lure him back to face charges.

Beijing has defended its decision to bar the three US citizens from leaving the country.

A foreign ministry spokesman told reporters that they "all have... valid identity documents as Chinese citizens" and are "suspected of having committed economic crimes".

On Thursday, Global Affairs Canada revealed that 13 Canadians had been detained in China since 1 December, although eight have since been released.

Image copyright EPA Image caption Meng Wanzhou, the chief financial officer of Huawei, was arrested in Canada at the request of the US

Among the Canadians who remain detained are former diplomat Michael Kovrig and businessman Michael Spavor.

Canadian teacher Sarah McIver who was reportedly released last week after she was held for "unlawfully working in China". China and Canada both said the case was different to that of Mr Kovrig and Mr Spavor who stand accused of harming national security.

China insists the detention of both men is not linked to Ms Meng's arrest, but many analysts believe it was a tit-for-tat action.

On Thursday, China's prosecutor general said the pair had "violated our country's laws and regulations" and were being investigated.