Updated at 12.15pm

An explosion went off at an office of the Afghan Voice news agency in the capital, Kabul, on Thursday causing serious casualties, officials and witnesses said, the latest in a series of attacks on media groups.

At least 40 people were killed and 30 wounded in the bomb attack, an Interior Ministry spokesman said.

However, the final toll may be higher, according to witnesses who said there were heavy casualties among students attending a panel discussion with researchers at the agency's offices.

Photographs sent by witnesses showed what appeared to be serious damage at the site and a number of dead and wounded on the ground.

There was no immediate claim of responsibility.

The attack, the latest in a series to hit Afghan media groups in recent years, follows an attack on a private television station in Kabul last month.

According to a report this month by media freedom group Reporters without Borders, Afghanistan is among the world's most dangerous countries for media workers with two journalists and five media assistants killed doing their jobs in 2017.

Afghan policemen stand guard at the site of a blast in Kabul. Photo: Reuters