Dissident Eritrean troops laying siege to the state's Ministry of Information have forced the director of state television to demand on-air the release of all political prisoners.

State media went off air after the call for prisoners to be freed, an intelligence official and diplomats in the region said.

About 200 soldiers with two tanks have surrounded the ministry in the capital, Asmara.

There was no immediate statement from the government.

One Western diplomat in neighbouring Ethiopia said other buildings might have been seized by soldiers too.

Accusing Eritrea of torture and summary executions last year, the United Nations human rights chief estimated that 5,000-10,000 political prisoners were being held in the country of about 6 million people.

Eritrea has been led by Isaias Afewerki, 66, for two decades, since it broke from bigger neighbour Ethiopia.

The gold-producing state is one of the most opaque countries on the continent and restricts access to foreign reporters.

Eritrean opposition activists exiled in neighbouring Ethiopia said there was growing dissent within the Eritrean military, especially over economic hardship.

Despite its relatively small population, Eritrea has Africa's second biggest army.

"Economic issues have worsened and have worsened relations between the government and soldiers in the past few weeks and months," one activist told Reuters.

Eritrea broke from Ethiopia in 1991. The two countries fought a 1998-2000 war over a border, which remains disputed.

Relations between them are strained, with Eritrea denying accusations it backs Ethiopian insurgents.

The UN Security Council imposed an embargo on Eritrea in 2009 over concerns its government was funding and arming al-Shabaab rebels in neighbouring Somalia - charges the government denied.