According to police, the blast occurred near the city's police headquarters, and may have been caused by a car bomb set off by a suicide bomber.
"At about 09:08 a.m. Moscow time [05:08 GMT] a yellow GAZelle truck broke through a gate at a check point near the building. A powerful explosion followed shortly after that," a local investigator told RIA Novosti.
"The building and cars parked around it are on fire," a police source earlier said.
Russia's mainly Muslim North Caucasus regions have seen a rise in violence in recent months. Attacks on police, officials and troops have been reported almost daily in Ingushetia and neighboring Dagestan that border Chechnya, which saw two separatist wars in the late 1990s-early 2000s.
Ingushetia's construction minister was gunned down in his office last Wednesday; the murder followed the killing of a Supreme Court judge and the attempted assassination of the republic's president, Yunus-Bek Yevkurov, in late June. Last month the Ingush forensic chief was also gunned down.
Russian President Dmitry Medvedev has called the increased terrorist activity in the Caucasus "an attempt to destabilize the situation in the region."