Five men escaped from the shelter and ran off. Officers gave chase, said Superintendent Jackie Ling Wai-po of the small boat unit, and fired several rounds of flares.

'To help colleagues on land, officers on two police craft at a nearby beach fired five rounds of flares to light up the area,' he said. 'But one of the flares was blown away by strong winds and it [seemingly] caused a small fire near a hilltop. We immediately alerted the Fire Services Department.'

After a brief chase, four mainland men aged between 18 and 26 were arrested. A fifth escaped.

Three officers and two of the mainlanders were injured during the chase and were taken to Tseung Kwan O Hospital.

The Information Services Department said the fire covered an area of 1km by 1.5km at about 7am when firemen called in the Government Flying Service. GFS operational manager Calvin Shum Chi-wai said five helicopters joined the operation, with one taking a team of firemen to the hilltop at 7am.

'Another four helicopters were engaged in making water drops between 8.30am and 12.30pm,' he said. 'The fire spread quickly over a vast area due to strong winds. At about 9am, there were two fire lines as long as 1km each.'

The Fire Services Department said the fire was put out at about 12.30pm.

Police said the mainlanders they were hunting in the six-hour operation were suspected of sneaking into Hong Kong to steal Buddhist pines in the country park.

The makeshift shelter, built with branches and nylon sheets and equipped with some cooking utensils, had been spotted by officers from the Sai Kung rural patrol unit on Wednesday.