Hongkongers have been warned by the security chief to be vigilant about their personal safety when travelling on the mainland. Ambrose Lee Siu-kwong's warning comes after two Hong Kong truck drivers said they were kidnapped in separate late-night incidents near the Huanggang border in Shenzhen last week. They were only released after their families paid $150,000. Mr Lee said Hong Kong officials were working with Guangdong authorities on the kidnappings. He admitted that the deteriorating law and order situation in Shenzhen had become a concern and urged visitors from Hong Kong to stay alert. 'People should not show off valuable items and should not take unlicensed taxis or minibuses,' he said. The Shenzhen Public Security Bureau has vowed to put more police officers on patrol near border areas. A bureau spokesman said the incidents were being taken seriously. 'Kidnapping is a serious crime and we will bring the offenders to justice. We have already deployed more officers on the border and we will make sure the personal safety of travellers is well protected,' the spokesman said. The city's worsening security has alarmed many visitors and investors. A string of crimes targeting Hong Kong people has been reported in recent weeks. Late last month, former Phoenix TV vice-president Zhou Yinan, and his family were found murdered in their Shenzhen home.