A suicide bomber car bomber blew himself up and wounded 25 policemen at a police station in northern Tajikistan on Friday, the interior ministry said.
An Uzbek militant group with links to al Qaeda was probably behind the attack, the ministry said.
Those who carried out the blast are most likely members of the Islamist Movement of Uzbekistan (IMU), it said in a statement.
Tokhir Normatov, chief of staff at Tajikistan's interior ministry, told Reuters it was not clear whether the bomber killed anyone else, but said the death toll was likely to rise.
The attack occurred in Khujand, some 340 km (211 miles) north of the capital Dushanbe and near the border with Uzbekistan. Tajikistan also has a porous 1,340-km (832 miles) border with Afghanistan where the IMU are also active.
Governments in the former Soviet republics of Central Asia are clamping down on what they see as growing radicalism in the predominantly Muslim, though secular, region after a rise in clashes between security forces and armed groups.
Tajik President Imomali Rakhmon last week urged parents to withdraw their children from religious schools abroad, arguing they could come home as terrorists.
Tajikistan suffered its first ever suicide bomb attack in January 2005, when a bomber struck the Emergencies Ministry in the capital Dushanbe, killing himself and wounding several people. Authorities blamed the IMU for that attack.
The country of seven million has jailed 115 people this year on charges of belonging to banned groups, mostly Islamic.
Last month, 10 followers of a banned Islamic group were jailed in Khujand for terms of three to 15 years.
Analysts say deepening economic hardship and social problems in Tajikistan, which relies heavily on remittances from migrants in Russia, push the population towards radical Islam.
(Writing by Amie Ferris-Rotman; Editing by Jon Hemming)