Guinea-Bissau's military said on Saturday it was ready to help the United States investigate drugs and arms trafficking between the west African nation and Colombia.

Forces spokesman Colonel Daba Na Walna promised "serious investigations if required by the government", but defended former army chief Antonio Indjai against US accusations of drug trafficking and seeking to sell arms to Colombian rebels.

"We cannot remain silent in the face of such accusations against the chief of staff. We require serious investigations to determine the guilty," he told a media conference in Bissau.

New York prosecutors announced Thursday that Indjai was facing four counts of conspiring to sell surface-to-air missiles to Colombia's FARC rebels to shoot down US patrol helicopters and of seeking to import huge amounts of cocaine into the US.

"If General Indjai is involved in trafficking jeopardising safety, the United States, like any other country in the world, will act for its safety," said Na Walna, who dismissed the "unfounded" allegations as an attempt to denigrate the image of the army chief and the country.

"We are open to serious inquiries that will lead to the arrest of real culprits, but not invented scenarios. We fear that invented plots will prevent the real culprits from being arrested."

The charges came less than two weeks after similar trafficking accusations were brought against Guinea-Bissau's former navy chief and four others arrested for an alleged trans-Atlantic plot.

Two other co-conspirators have been caught in Colombia and are awaiting extradition to the United States.

It was not immediately clear whether Indjai, now the eighth person from Guinea-Bissau targeted by the US authorities, remained at large or whether he was in US detention.

The charges result from undercover sting operations that began in August and culminated in dramatic arrests of some of the accused by US agents on a boat in international waters off West Africa earlier in April.

Indjai, a former army chief of staff, led a coup in April 2012 that ousted the regime of former premier Carlos Gomes Junior.

He agreed in May last year to hand power to a civilian transitional regime headed by President Manuel Serifo Nhamadjo, who had been due to hold elections within 12 months. But that plan has now been postponed.

Guinea-Bissau, a country of just 1.5 million people, has suffered chronic instability since independence from Portugal in 1974 due to conflict between the army and state.