Guinea-Bissau security forces arrested several suspects over a foiled coup bid in the chronically unstable nation, which the government has accused former colonial ruler Portugal of instigating.

The transition government in the west African nation, which was itself set up after a successful coup in April, insisted it was in control after an attack on an elite army unit barracks on Sunday which left at least seven people dead.

Two politicians and a retired military officer were arrested Monday, security sources said, while the hunt continued for more coup plotters and the man believed to be their leader, Captain Pansau N'Tchama.

A military source said the country's northern borders with Senegal were being "strictly controlled".

"Some fugitives could cross into this neighbouring country. We have put all our units on alert," the source said on condition of anonymity.

Prime Minister Rui de Barros vowed a "transparent and impartial" investigation.

N'Tchama was the head of a commando unit that assassinated president Joao Bernardo Vieira in 2009. He returned last week from Portugal where he had been undergoing military training since July 2009, security sources said.

"What is strange to us is how Pansau, who was in Portugal as a political refugee, could return so easily," the prime minister told diplomats on Monday.

"We have information that infiltrators provided money to some soldiers. Investigations will tell us more about this matter," he said.

"Our duty is to bring the necessary explanations to the international community about Sunday's attack. Because there are elements who were arrested who have told us the plot was carried out with the help of a foreign nation."

On Sunday the government named former colonial power Portugal as being behind the raid, labelling it a coup attempt.

"The government considers Portugal, the CPLP (the Community of Portuguese Language Countries) and Carlos Gomes Junior as the instigators of this attempt at destabilisation," said a statement read by Communications Minister Fernando Vaz on Sunday.

Its aim was "to overthrow the transitional government... undermine the political process that is under way with one objective, to bring Carlos Gomes Junior back to power and justify an international stabilisation force", it said.

It was not immediately clear why N'Tchama might have carried out the assault, but the captain is also a former associate of the government overthrown in the April coup.

That putsch toppled the government of Carlos Gomes Junior, interrupting a presidential election between the first and second rounds, which he was leading after the initial vote.

The seven dead included six gunmen and a soldier, according to an AFP tally.

Three three arrested Monday are believed to have ties with Gomes Junior.

Lisbon on Monday called for "calm and an end to the violence" in Guinea-Bissau, but declined to comment on the accusations that it was behind the attack.

The United Nations also condemned the attack in a statement Monday and appealed on Guinea-Bissau to "resolve differences by peaceful means, including through inclusive dialogue."

Since independence from Portugal in 1974, the army and state in the nation of 1.6 million people have remained in constant conflict, leading to chronic instability which has allowed drug trafficking to Europe to flourish.

No president has ever completed a full term in office.

The April 12 coup was led by army chief General Antonio Indjai in protest at the strong presence of Angolan troops in the country which were seen as a threat to Guinea-Bissau's own armed forces.

His junta later handed power to a transitional government, but the coup leaders remain influential: it was they who chose Manuel Serifo Nhamadjo for the role of interim president.

In a speech last month however, Nhamadjo insisted that his government was "not under the army's orders, either in form or substance". He called on the international community to back the fight against drug trafficking and help organise elections in 2013.

After the April coup, the European Union, the country's chief trading partner, suspended aid and imposed sanctions on a number of military officers, including Indjai.

But the West African bloc ECOWAS has recognised the transitional administration.