The African Union said it was deeply concerned by the attacks by soldiers late Thursday on the homes of acting President Raimundo Pereira and Prime Minister Carlos Gomes Junior in the city of Bissau.
The whereabouts of the country's interim president and prime minister are not currently clear.
A statement put out by the group behind the coup bid, calling itself the Military Command, said the revolt was in response to a "secret deal" between the governments of Guinea-Bissau and Angola.
This "deal" was drawn up by the Guinea-Bissau government to allow Angolan troops in the country and mandates "Angola to attack Guinea-Bissau's military," a communique from the group said.
The group claims Pereira and Gomes signed the deal, and accuses the African Union, whose rotating presidency is currently held by Angola, of also supporting intervention by Angolan forces.
In another statement, the Military Command said it has no ambition to "stay in power."
Guinea-Bissau's history has been marked by military coups and attempted coups since the nation of 1.6 million gained independence from Portugal in 1974. The conflicts ravaged the West African nation's infrastructure and economy, leaving it among the poorest in the world.
It is not yet known who is leading the latest coup attempt.
Lt. Col. Dabana Na Walna, the spokesman for Guinea-Bissau's armed forces, told CNN he does not speak for the group. The chief of staff of Guinea-Bissau's armed forces is Gen. Antonio Injai.
African Union Chairman Jean Ping condemned the army's "blatant acts of indiscipline" and called on the military personnel involved to honor their commitment to serve the constitutional government.
Guinea-Bissau recently held the first round of voting in its presidential election, with campaigning for the second round about to begin. The election was prompted by the death in January of the incumbent president.
The residence of Gomes Junior, the prime minister and also a presidential candidate, was looted, witnesses said, as were the homes of some of his ministers, including Interior Minister Fernando Gomes.
Another communique from the Military Command called for calm and advised people in the capital "to refrain from acts of violence and vandalism that could jeopardize the order."
The country's public media outlets are under the control of the army and are regularly broadcasting statements from the military, witnesses said Friday.
A number of Angolan troops are present in Guinea-Bissau under a bilateral agreement, on a mission to help reform the country's armed forces, Angola said.
Angola, also a former Portuguese colony, issued a statement earlier this week stating its intention to unilaterally withdraw its troops.
The U.S. Embassy in Senegal said the situation in Guinea-Bissau is unclear, but it is closely monitoring events. It urged U.S. citizens to remain where they are and avoid downtown Bissau.
"We are disheartened by the negative turn of events so soon after the first round of Guinea-Bissau's presidential election. We are deeply concerned about the safety of all those in Bissau today, and urge all parties to put down their weapons and restore legitimate civilian leadership."
Elisabete Azevedo-Harman, a professor of politics at the Catholic University of Portugal and Mozambique currently in Bissau, told CNN she heard gunfire and explosions Thursday night, chiefly in the area of the prime minister's residence.
Soldiers and police remained on the streets of the capital Friday, but in smaller numbers than the night before, she said. People were starting to venture out again, she added.
President Malam Bacai Sanha, 64, died in January after a long illness, triggering the presidential election. The first round of voting in March did not produce a clear winner.
The nation's former head of intelligence was shot dead in the capital hours after the first round concluded, but the vote was otherwise peaceful.
Sanha became president in September 2009, months after the assassination of his predecessor.
Despite his coming to power in what international observers deemed a fair and peaceful election, his tenure was marked by turmoil among the country's military and political leadership.
To date, no democratically elected president has served a full, five-year term.
