DHAKA, July 2 (Reuters) - A senior Denmark security official has joined Bangladesh forces working to rescue a kidnapped Bangladeshi working for Danish development agency (Danida), police said on Monday.

The Danida staffer, Suman Sharif, was kidnapped along with his driver on June 25 at a village near Thanchi, some 400 km (250 miles) southeast of the capital Dhaka. The motive was unknown.

In the wake of a massive hunt for the abducted men, kidnappers released the driver on Thursday at a remote forest near Thanchi township.

"The government security official from Denmark has started evaluating our efforts to rescue the Danida staff. He has also exchanged (knowledge of) his experiences in such situations," a senior administrative officer said.

Thanchi is a township in the Chittagong Hill Tracts (CHT) adjacent to Tripura and Mizoram, which are eastern states of India, and Arakah (Rakhaine) state in western Myanmar.

Various rebel groups have been active in these states for decades on different issues, intelligence sources said.

Bangladesh security forces earlier detained some 30 people, including 10 Myanmarese without valid travel documents, for suspected involvement with the kidnapping.

Officials said some 600 security force members including commandos were scouring the forests.

The Danida staff had no security escort, which visitors need in remote areas in the CHT, previously the scene of a tribal insurgency, officials 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 rebels for more than a month at Rangamati forest in the CHT in 2001 before being released. It was not known whether ransom was paid for their release.

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

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