Image copyrightAFPImage 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 warns that US citizens have been arbitrarily prevented from leaving the country.The warning comes as two Canadian citizens remain in detention in China.Former diplomat Michael Kovrig and businessman Michael Spavor were arrested last month as relations between the two countries worsened.The pair face accusations of harming national security and, on Thursday, China's top prosecutor said they had "without a doubt" violated the law.Separately, 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.What is the latest travel adviceThe new advisory warns of so-called exit bans which prohibit foreign citizens from leaving China.It says the bans have been used "coercively" to "lure individuals" back to the country.

It also adds that US citizens have been detained for years and subjected to harassment while under an exit ban."US citizens may be detained without access to consular services or information about their alleged crime," the advisory reads."Individuals not involved in legal proceedings or suspected of wrongdoing have also been subjected to lengthy exit bans in order to compel their family members or colleagues to co-operate with Chinese courts," the state department said in a separate warning issued last January.The latest advice also warns of "special restrictions" on those who hold dual US-Chinese citizenship.Dual-citizenship is not allowed under Chinese law, and the state department has warned that US-Chinese nationals can be detained and denied US assistance in China.It advises travelling on a US passport with a valid Chinese visa and asking officials to notify the US embassy immediately if you are detained or arrested.What do we know of the recent detentionsImage copyrightEPAImage caption Meng Wanzhou, the chief financial officer of Huawei, was arrested in Canada at the request of the US On Thursday, Global Affairs Canada revealed that 13 Canadians have been detained in China since 1 December, which is when Huawei chief financial officer Meng Wanzhou was arrested in Vancouver at the request of US prosecutors.Eight of them have since been released.That number includes Mr Kovrig and Mr Spavor but also 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.Meng Wanzhou faces extradition to the US to face fraud charges, which she denies, that are linked to allegations of avoiding US sanctions on Iran.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.Beijing has also defended its decision to bar the three US citizens from leaving the country in November.A foreign ministry spokesman told reporters that they "all have valid identity documents as Chinese citizens" and are "suspected of having committed economic crimes".Their father, 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.