An explosive device went off during May Day celebrations in Russia's restive North Caucasus region of Kabardino Balkaria, killing one man and injuring 21. Interfax says the victim who died in hospital, was a 104-year-old veteran of World War II.

The bomb exploded around 12.45 pm (0845 GMT) in a hippodrome during horse races in the capital Nalchik, the investigative committee of prosecutors said in a statement.

"On the roof of the canopy at the hippodrome located in the city of Nalchik, an unknown type of explosive device was set off. As a result, five to six people were injured. Two of them were severely wounded," it said.

Russian news agencies quoted police as saying one person had died from his injuries, adding that 21 people were wounded.

Russian President Dmitry Medvedev ordered a federal probe into the latest attack in the restive region.

Kabardino-Balkaria is a predominantly Muslim region of Russia's volatile North Caucasus which has occasionally experienced militant attacks, though it has mostly avoided the large-scale violence of nearby Chechnya or Dagestan

But attacks have spiked in recent months. On Friday, four people including two police officers were injured in a bomb blast in Nalchik.