"It's true that the intelligence community -- national and military security agencies -- are investigating what looks like a serious breach of security on the part of some military officers and apparent civilian collaborators," said President Olusegun Obasanjo's spokeswoman, Remi Oyo.

Asked to confirm reports from military sources that 28 officers had been detained, interrogated and released within recent days as part of the probe, Oyo said: "I cannot say the exact figure, but it's a considerable number."

Officials played down fears that Africa's most populous country was at risk of its sixth military takeover since 1966, insisting the officers concerned posed no threat to Nigeria's five-year-old experiment with democracy or to Obasanjo's continued rule.

But any hint that the world's sixth largest oil exporter might face another round of instability will raise concerns in the international community, which has come to see the country as a source of much needed stability in its war-torn region.

Nigeria won its independence from Britain in 1960, suffered the first of many military power grabs six years later and has since endured a total of 28 years under various military rulers, including a brutal 30-month civil war.

Despite having Africa's richest oil fields, corruption and military misrule have crippled the economy. In 30 years, the proportion of people living in abject poverty on less than a dollar a day has doubled, to almost 80 percent.

Nigeria's 126-million-strong population is divided into 250 tribal groups and two major religions -- Islam and Christianity -- and religious and ethnic unrest has killed at least 10,000 people since civilian rule returned in 1999.

Some reports said that most of the officers under investigation are from the Hausa people, one of the three largest linguistic groups and the dominant culture in Nigeria's mainly Muslim north.

In Kano, a northern city, an army captain said: "It's true that arrests have been made in military circles of officers accused of attempting to subvert the government, but military intelligence is keeping the issue very confidential.

"We don't know what this is leading to," he said, on condition of anonymity.

An army general, also speaking confidentially, told AFP that most of those questioned were disgruntled elements without current commands and, while being potential troublemakers, were no threat to the government.

The general said that Wednesday's transfer of Hamza al-Mustapha, the former security chief of late military dictator Sani Abacha, from a civilian Lagos prison to a military intelligence facility had been connected to the probe.

The Nigerian Prisons Service said on Thursday in a statement that al-Mustapha had been released to agents of the Directorate of Military Intelligence for questioning on "matters of national security".

Al-Mustapha, who was seen as Abacha's right-hand-man during his 1993-1998 reign, was on remand in Kirikiri prison on a charge he ordered the attempted murder in 1996 of a newspaper publisher who fell out with the regime.

The detainee's brother Hadi alleged that he was shot in the leg during his transfer, a claim denied by the police, and told AFP: "This coup allegation is just a ploy to keep him behind bars, and ultimately kill him."

This week, Nigerian authorities banned broadcasters from re-transmitting live news from foreign sources such as the BBC and CNN, which regulators said was "not in the national interest".

It was not clear if the ban was linked to the coup rumours.

Despite its chequered past and unruly present, Nigeria is considered by many international observers as key to preserving stability in west Africa. Its troops have won praise for peacekeeping missions in Liberia and Sierra Leone.

Last year foreign visitors, including US President George W. Bush and Britain's Queen Elizabeth II, welcomed the country's 1999 return to elected rule and subsequent progress towards entrenched democracy.

The army last relinquished power in 1999 with the election of a former military ruler, Obasanjo, as civilian head of state.

He was re-elected in April last year, in a poll which both Nigerian and international monitors said was marred by widespread ballot-rigging.

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