An explosion has hit a bus carrying presidential guards in Tunisia's capital, Tunis, killing at least 12 people, officials say.

President Beji Caid Essebsi has declared a 30-day state of emergency and the capital is under curfew.

No group has yet said it was behind the attack.

Tunisia has been targeted by the Islamic State (IS) group, including an attack by a gunman on the beach resort of Sousse in June, killing 38 people.

The North African state is believed to be the biggest exporter of jihadis, with the authorities saying at least 3,000 of its nationals are fighting in Iraq and Syria.

The explosion happened at a bus stop where the presidential guard picks up and drops off its staff, near the former headquarters of the party of deposed President Zine El Abidine Ben Ali.

Roads in the city were already clogged because of heavy rain and flooding when the explosion hit.

Exactly what caused the blast is unclear but one source told Reuters a bomber had probably detonated explosives in the vehicle.

For many people here, an attack like this was only a question of time. It has been a deadly year for Tunisia's foreign visitors, who were targeted by militant gunmen.

Now, their security services have been hit in the heart of the capital. The army is sporadically engaged in deadly battles with some militants in more remote areas of the country, but a bombing of this kind is a first in Tunis.

In recent months, there has been a more visible police presence in sensitive areas, including where tonight's explosion took place. Tunisia has been increasingly cooperating with counter-terrorism advisors from the UK and the US this year.

However, the mistrust between different local security agencies, and what some Western diplomats privately describe as a chaotic state of affairs within their ranks, meant that they were still very vulnerable.

Mr Ben Ali was ousted in a popular uprising in 2011.

Tunisia currently has a secular government, which is battling the militants.

In March, gunmen attacked the famous Bardo Museum in Tunis, killing more than 20 people.