Rights groups have condemned China after a Beijing-based journalist went missing, linking his disappearance to an unusual open letter calling for President Xi Jinping's resignation. Jia Jia, a freelance journalist, has not been seen since Tuesday, his lawyer told AFP, without giving further details. Amnesty International said a close friend of Jia told the group he disappeared some time after going through customs at Beijing airport when about to board a flight to Hong Kong. "He went missing on the 15th," lawyer Yan Xin said, citing the journalist's wife. City University of Hong Kong also confirmed to AFP that Jia had not turned up to a seminar he was due to give on Thursday. "We are deeply concerned by Chinese journalist Jia Jia's disappearance," said Bob Dietz, Asia program coordinator for the Committee to Protect Journalists. "If he is in police custody, officials must disclose where they are holding him and why. If anyone else knows where he is, they should step forward and clarify this worrisome mystery." Under Xi, China's ruling Communist Party has tightened controls over civil society, detaining or interrogating more than 200 human rights lawyers and activists last year in what analysts have called one of the biggest crackdowns on dissent in recent times. Sophie Richardson, China director for Human Rights Watch, voiced concern over Jia on Twitter. "#China disapps journo--no longer enough to just erase all trace of criticism. Trend now is to erase critics, too," she tweeted. Both Amnesty and the CPJ have linked Jia's disappearance to an open letter published on the news website Wujie News earlier this month calling for Xi's resignation. The letter, which was rapidly removed, was signed "Loyal Communist Party members", but little else is known about its authorship. "His going missing is most likely related to the publishing of the letter and perhaps the authorities' implication of his involvement or knowledge of the letter," Amnesty China researcher William Nee told AFP. "Journalists and activists are forced all the time to 'drink tea' with the authorities... but it generally doesn't last this long," he said, adding that officials usually try to extract information during such meetings. However, Jia's lawyer Yan said his disappearance may not be connected to the letter. China is currently in the spotlight over the disappearances last year of five Hong Kong booksellers who reappeared on the mainland, as well as the use of televised confessions from suspects. The five booksellers were from Hong Kong's Mighty Current publishing house, known for its salacious titles critical of Beijing.