(adds release of one)

DHAKA, June 28 (Reuters) - Suspected Bangladeshi tribal rebels released on Thursday one of two kidnapped Bangladeshi staff working for a Danish development agency, officials said.

Security forces, scouring forests around the Chittagong hills, arrested five tribal people from the spot where Mohammad Hanif, a driver of the Danish International Development Agency (Danida), was found lying unconscious.

They had earlier detained a dozen people in connection with the kidnappings of Hanif and Suman Sharif, another Danida employee.

Hanif was taken to hospital.

The Danida staff were captured on Monday by suspected tribal rebels at a village near Thanchi, some 400 km (250 miles) southeast of the capital Dhaka.

The motive behind the abductions was unknown.

Helicopters dropped commandos into dense forests on Thursday, witnesses said, as part of an operation to find the aid workers.

"We are confident we will recover them unharmed. There will be no respite in the operation unless they are freed," senior government administrative officer Mokhlesur Rahman said.

The Danida staff had no security escort, which visitors need in remote areas in the Chittagong Hill Tracts (CHT), previously the scene of a tribal insurgency, he said.

Tribal insurgents seeking autonomy for the 14,000 sq km (5,500 sq mile) CHT fought a guerrilla war against Bangladeshi authorities from 1973 until 1997, when a peace deal was struck.

The majority of the 3,000-strong Shanti Bahini rebels surrendered their arms, but a dissident group opposed to the deal melted into the jungle and turned to criminal activities, including kidnappings for ransom.

A British and a Danish engineer working on a road project were detained by suspected tribal rebels for more than a month at Rangamati forest in the CHT in 2001 before being released.

It is not known whether a ransom was paid.

More than 8,500 people were killed and some 70,000 were displaced during the insurgency.

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