Chad's Foreign Minister Mahamat Zen Cherif, in a visit to Equatorial Guinea after a bid to overthrow long-time President Teodoro Obiang Nguema, condemned the attempted coup as a "major threat" to central Africa.
"The attempt at destabilisation is not just an affair that only concerns Equatorial Guinea, it is also a major threat of destabilisation that concerns the entire sub-region of central Africa," he said in remarks reported by state television TVGE.
On Wednesday, Security Minister Nicolas Obama Nchama said the authorities had thwarted an attempted coup, allegedly mounted on December 24 by foreign mercenaries recruited by political opponents.
Hours after his statement, TGVE reported clashes with "mercenaries" near the border with Cameroon.
Government troops shot dead one "mercenary" and "used gunfire to disperse (others) in the forests along the border", it said, without specifying how many fighters were involved or how long the clashes lasted.
Formerly a small Spanish colony, Equatorial Guinea has become one of sub-Sahara's biggest oil producers but a large proportion of its 1.2 million population lives in poverty.
Obiang, 75, is Africa's longest-serving leader, having seized power in 1979.
Critics accuse him of brutal repression of opponents, electoral fraud and corruption. He has faced a string of coup attempts in his more-than 38-year tenure.
Cherif, who was cited by state media after he spoke with Obiang on Wednesday, called on "all countries in the sub-region to unite their efforts to deepen inquiries, not just to understand what happened here but also to establish the cause of this attempt at destabilisation."
He said he would go to Cameroon for talks with leaders there.
According to Wednesday's statement, the mercenaries were Chadian, Sudanese and Centrafricans, as citizens of the Central African Republic are called.
"They infiltrated Kye Ossi, Ebibeyin, Mongomo, Bata and Malabo to attack the head of state, who was in the Koete Mongomo presidential palace for the year-end holiday," it said.
The "mercenaries... were recruited by Equatorial Guinean militants from certain radical opposition parties with the support of certain powers", the minister said.
The attempted infiltration had been repelled with the help of the Cameroon security services, according to the authorities.
Sources told AFP that the country's ambassador to Chad, Enrique Nsue Anguesom, who was on holiday in Equatorial Guinea's Ebibeyin district, had been arrested and was being held in a military camp.
Cameroon says that on December 27 it arrested 38 heavily armed men on its border with Equatorial Guinea, which consists of mostly dense forest territory on the African mainland and an offshore island where the capital lies.
Cameroonian security sources on Thursday said those arrested included an ex-general in the Chadian army, Mahamat Kodo Bani, who was once a senior officer in the presidential guard.
He is being held in Yaounde, the Cameroonian capital, they said.
In Gabon, which also borders Equatorial Guinea, a senior government official said "security measures" had been taken on the border.
"We are working in coordination with Cameroon and Equatorial Guinea," the source said.
Facebook, Whatsapp and VPNs in Equatorial Guinea have been blocked. "There is a real lack of transparency on what's really going on," a diplomat in the region said.
Obiang took power in a coup on August 3, 1979, ousting his own uncle, Francisco Macias Nguema, who was shot by firing squad.
He was re-elected to a fifth seven-year term in 2016, gaining more than 90 per cent of the vote according to the official results.
Legislative elections on November 12 last year saw the ruling party win 92 per cent of the vote, a result condemned as fraudulent by dissidents.
The Citizens for Innovation (CI) opposition group on Wednesday strongly denied it had played any part in the attempted coup.