AN urgent review of border security is likely after a police sergeant alone on patrol was shot dead by a suspected Chinese illegal immigrant yesterday. Sergeant Tai Choi, 43, was shot twice in the thigh at close range and left to bleed to death in a vegetable patch. There was evidence that Sergeant Tai had stopped the man while he was patrolling around Nga Yiu Ha Tsuen, Ta Kwu Ling, in a police van. They apparently struggled and Sergeant Tai was shot, without being able to fire his own revolver. Sergeant Tai, who was standing in for a colleague who had taken the day off, is the second policeman to be killed while on duty this year. Three officers were killed last year. The Deputy Secretary for Security, Mr James Morris, said the Government had identified several black spots of illegal immigration, including Lau Fau Shan and the Castle Peak area. He said police would take up the case with the Chinese Government through the border liaison unit. ''If this is related to an illegal immigrant carrying arms, they will talk about it,'' Mr Morris said. ''They have constant contact and they would surely discuss it if the case involves firearms.'' He said the Security Branch was examining options to upgrade the 34-kilometre border fence. Nearly 14,000 illegal immigrants have been caught this year. The figure for 1992 was 33,703. Mr Morris said the Government was keen to close the estuary of Yuen Long Creek and would work with the Planning Department to avoid damage done to the vulnerable drainage there. Although the main section of the fence running from Shataukok to the east of the creek was upgraded several years ago, Mr Morris said the police were looking for ways to plug holes cut open by illegal immigrants. Stronger wire may be installed to make it more difficult to force entry. Mr Morris said the section of the border fence west of Yuen Long Creek would be reinforced. The last upgrade in 1989 cost $16.5 million. At about 9.30 am yesterday, a farmer from Nga Yiu Ha Tsuen heard a man shouting for help and the sound of a gunshot. He found Sergeant Tai lying near a stream. The officer was taken to Fanling Hospital where he was certified dead at 10.20 am. The suspect, aged between 30 and 40, was seen running from the area. The Police Tactical Unit and a helicopter were deployed in the search for him and police wearing bullet-proof vests and carrying rifles set up roadblocks. By late last night no one had been detained. The search, however, did turn up the decomposed body of a man near the scene of the shooting. Initial examination by the force pathologist revealed the man had been dead a few days. New Territories North Regional Crime Unit officers are investigating the case, but it was unlikely there was any connection between the two incidents. Sergeant Tai is survived by his wife, a son and two daughters - one of whom is a policewoman, according to a friend. A brother-in-law, a Mr Cheung, said the family felt miserable even though they had always accepted there was an inherent risk in police work. Villagers living in Nga Yiu Ha Tsuen expressed fear that more illegal immigrants were being seen in the neighbourhood. A villager, Mrs Wong Lee So-fung, said the illegal immigrants were usually in groups of two or three. ''We are really living at risk, even though they have not caused any trouble here so far,'' she said. ''We are on alert and keep doors and gates closed at all times.'' Mrs Wong, who moved into the village with her husband and nine-month-old daughter last September, said there were about 10 households. Another villager said illegal immigrants always showed up at night and she was alerted by her dog barking. She said the police seldom patrolled the village at night. Legislative Council security panel member Mr James To Kun-sun last night called on police to determine whether the incident could have been avoided. He also asked the police to reconsider the strategy of allowing policeman to patrol alone. ''Even in quiet spots of the urban areas, policemen carry out their duty in pairs,'' he said. Mr To said police should be on alert when they encountered illegal immigrants, even though weapons were seldom found on them. He said there was no reason to increase police patrols in remote villages near the border, because the shooting was an isolated event. ''In general terms, mainland illegal immigrants do not hold-up or threaten villagers with weapons and they only ask for food and water,'' he said. Mr Morris described the incident as tragic. ''Most of the illegal immigrants are not armed, not on their entry. They would sometimes carry knives,'' he said. The Secretary for Security, Mr Alistair Asprey, will visit the border today to inspect measures being taken to stop people crossing illegally. He will meet officers of the Royal Hongkong Regiment, which is doing border patrol duties for a week. The regiment has been deployed to help police in areas where there have been serious breaches of security. Mr Morris said it was wrong to assume that the regiment had taken on a dangerous assignment in patrolling the border, although they were not armed. ''Most illegal immigrants are not armed and it is not right to say that the duty at the border is particularly dangerous,'' Mr Morris said.