Nigerian troops have tightened security around the presidential villa in the capital, Abuja, without explanation, amid uncertainty over the health of the military leader, General Sani Abacha.

Journalists who'd been summoned to the his compound were kept out by soldiers and have remained outside waiting for further details.

General Abacha, who is 54, has made few public appearances since greeting Pope John Paul in March, and last week failed to attend a special function in Lagos.

The general seized power in 1993 after cancelling presidential elections.

He has promised to hand over to an elected civilian administration, but in April he became the only nominated candidate for the presidency.