A bomb has exploded under a car in Belfast, the capital of Northern Ireland but no one was injured, while part of a major train line has been suspended in a separate security alert, police said on Thursday.

In a further two incidents, police were investigating suspicious objects in separate places in Belfast and have evacuated nearby premises.

"We believe at this stage that a device may have exploded, or partially exploded under a parked car, causing extensive damage to the car," the Police Service of Northern Ireland said in a statement on an incident late on Wednesday in St James Road.

Part of main train line between Belfast and Dublin, the capital of the Republic of Ireland further south, has been suspended all morning due to a security alert, police said.

The threat level for Northern Ireland-related terrorism is currently set at severe, meaning an attack is considered highly likely.