Civil aviation authorities said the helicopter was hit four times but managed to return to its base with the crew unharmed.

Londono: A Harvard-trained economist

It is not clear who carried out the attack, although the area where the plane disappeared is controlled by left-wing rebels from the Revolutionary Armed Forces of Colombia (Farc).

The helicopter and two military aircraft had gone out at dawn to resume the search for a plane carrying Social Welfare Minister Juan Luis Londono and four other people, which disappeared over the Tolima province near the central Andes mountain region, about 100 miles (60 kilometres) west of the capital, Bogota.

Rescue efforts have been hampered by the inaccessible terrain of the area being searched, near the mountain towns of Calarca and Cajamarca.

Colombian President Alvaro Uribe has cancelled all engagements while he waits for news of the minister's whereabouts; he also visited the minister's family on Thursday night, French news agency AFP reported.

'Humanitarian operation'

The minister's plane was reported missing on Thursday, but aviation officials said that no emergency calls were received from the craft.

Flying conditions were reportedly good, although Mr Londono's driver told local radio that the pilot had experienced trouble in starting the plane's engine's before take-off, Reuters news agency reported.

Correspondents say there is so far no evidence his aircraft was shot down.

However Air Force officials said there had been recent fighting in the area between the army and Farc guerrillas, the French news agency AFP reported.

Civil aviation director Juan Carlos Velez appealed for armed groups in the area to allow the search to continue unhindered.

"Military planes are taking part but it is a strictly humanitarian operation, and we requested the civilian population and especially the armed groups to help us," Mr Velez was quoted as saying.

'Flares sighting'

Mr Londono's twin-engine plane failed to make normal contact shortly after taking off on Thursday afternoon from an airport in Girardot, some 80 kilometres (50 miles) southwest of the capital, Bogota.

He was travelling to the city of Popayan, some 370 kilometres (230 miles) from Bogota.

Peasants in the mountainous region about 100 miles (60 kilometres) west of Bogota reported hearing an explosion on Thursday afternoon.

However others reported seeing flares, suggesting there might be survivors, rescue officials said.