A state of emergency remains in force in Bangkok and other provinces months after deadly street protests [Reuters]

At least three people have been killed and several others injured after an explosion hit a residential building north of the Thai capital, Bangkok.

The blast occurred at about 6pm local time (11:00 GMT) in the Bang Bua Thong district of Nonthaburi province on Tuesday, a local police official said.

"We still don't know the cause of the explosion. Two storeys of the mansion collapsed," Colonel Somchai Hussakul, a police spokesman, said.

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He said the blast probably occurred in a room in the building.

The explosion came just hours after Thailand's government extended a controversial state of emergency in Nonthaburi and three other provinces, including Bangkok.

The emergency was imposed in the wake of deadly street protests and a series of grenade blasts in the capital.

A string of small bombings, thought to be politically motivated, have hit Bangkok and the surrounding areas this year.

Authorities recently gave warning that increased violence was expected beginning this month, and over the weekend 11 men were detained in the northern province of Chiang Mai.

They are suspected of training for armed attacks on behalf of a movement to overthrow Thailand's monarchy.

The state of emergency came into force in April when at least 90 people were killed and almost 2,000 injured in clashes between the military and the anti-government red shirts.

Source: Agencies