A defence ministry document gave no reason for the unprecedented move apart from stating that it was to "safeguard the supreme values of the motherland".

The alert came into force on Thursday.

The navy has stepped up its patrols, partly to ensure the safety of offshore oil platforms.

The police and paramilitary gendarme force have also increased patrols and the customs service was carrying out checks on everyone entering the country.

The only recent security incident of note was the abduction by pirates of seven sailors off the country's coast on Wednesday.

Equatorial Guinea, Africa's only Spanish-speaking nation, is the continent's third-biggest oil producer but more than half of its population live below the poverty line.

It is regularly criticised by human rights groups, not just for its corruption but for its repressive laws, and is accused of carrying out unlawful killings and torture.

Towards the end of 2017, security forces arrested some 30 armed men from Chad, Central African Republic and Sudan after they entered the country from Cameroon for what Equatorial Guinea says was a foiled coup attempt.