Sources said officials of police and paramilitary BDR conducting the operation from three sector headquarters of Pirojpur, Satkhira and Jhenidah had parleys for three days to finalise the strategy to mop up the criminals.

"We will go for block raid, search every house," said an official requesting anonymity.

Despite secrecy maintained by the operation leaders, movement of forces has alerted the underground activists and criminals who have gone into their hideouts. Many of them may have crossed the border, said villagers.

Rupsha upazila of Khulna and Fakirhat and Mollarhat upazilas of Bagerhat are the most troubled areas where about a dozen policemen fell victim to underground activists in recent months.

UNB Satkhira correspondent said more than 2,000 personnel of the joint force camped at nine centres in the town last night to launch operation against the underground criminals.

Sources said as the news of arrival of the joint force here on Thursday spread in the town and trickled down to remote villages, criminals went into hiding, many in shrimp enclosures or across the border.

Local BDR commander Lt Col Mahbubur Rahman said they had taken adequate measures so that the criminals could not cross the border.

Officials said the joint force stationed here was drawn from six districts including Dhaka and Khulna.