A female suicide bomber blew herself up in front of policemen in Russia's Dagestan on Saturday, injuring 18 people in the second bombing attack to hit the restive region this week, officials said.
Two of the victims -- both policemen -- were in a critical condition, police said.
The attack took place not far from the interior ministry building in the centre of Makhachkala, interior ministry spokeswoman Fatina Ubaidatova told AFP.
An unidentified woman came up to traffic police and detonated an explosive, according to the Moscow-based Investigative Committee.
"Two policemen who were closest to the suicide bomber are in an extremely serious condition," the regional police said in a statement.
The force of the explosion was around 500 grams in TNT equivalent, the statement added.
Feizulakh Gabibulayev, first deputy health minister for the region of Dagestan, told AFP that 18 people had sought medical assistance, and that 15 were hospitalised.
Five among them were in a serious condition, he added.
State television meanwhile broadcast a picture of a young woman, her head covered with a scarf, suggesting that she might have been behind the bombing.
The latest attack in Russia's troubled Northern Caucasus region comes after twin car bombs killed four people and wounded more than 40 in the same city on Monday.
Dagestan is one of Russia's most violent regions.
It experiences almost daily shootings and bombings that officials blame on local criminals and Islamists with links to Chechnya, where over the past 20 years the Kremlin has fought two wars.
The region is also home to the parents of Boston marathon bombing suspects Dzhokhar and Tamerlan Tsarnaev.