Mainland officials have failed to provide details of an allegedly covert operation this month in Hong Kong for which two mainland Public Security Bureau officers and five other men were arrested, the city's security chief said yesterday. Addressing the security panel, Secretary for Security Ambrose Lee Sui-kwong said he had phoned the head of the Guangdong bureau requesting co-operation with the Hong Kong police investigation. But so far there had been no response on the identity of those arrested and the purpose of their visit to Hong Kong, Mr Lee said. The seven men - one carrying a pair of handcuffs - were arrested while apparently conducting a surveillance operation outside Cape Mansions in Mount Davis Road on June 16. They were detained after a phone call from a member of the public who reported that the seven were acting suspiciously. They have not been charged and have been released on bail. Police declined to confirm if the seven were still in Hong Kong. Mr Lee told the panel that the mainland authorities had expressed serious concern over the incident, but had not offered any explanation. He said an investigation was under way to determine if the men had committed any offences, as well as checking if there had been a breach of the security arrangement between Hong Kong and the mainland. Releasing figures yesterday, Mr Lee said formal requests for police assistance between the mainland and Hong Kong were becoming more frequent. Public Security Bureau officials have asked for support from the Hong Kong police in 35 cases involving 101 suspects in the first four months of the year. In the whole of last year, this same request was made in 127 cases involving 414 suspects. Lawmakers yesterday criticised 'stonewalling' tactics being used by the mainland authorities, saying the case needed to be taken up at the highest level. 'I think we have a huge loophole,' Democratic Party legislator Cheung Man-kwong said. 'This is a severe blow to Hong Kong and should not be allowed to recur. We need to review grey areas and plug any loopholes to prevent mainland law enforcement officials operating freely in Hong Kong.' He said the men should face stiff penalties for breaking the law in Hong Kong. Legislator Margaret Ng Ngoi-yee said the men should also face immigration offences.