Up to 20 soldiers in Papua New Guinea say they have taken control of the military and are demanding the reinstatement of the ousted PM.
The apparent mutiny took place early Thursday in Port Moresby, reports said.
It appeared to be linked to the conflict between Peter O'Neill and Sir Michael Somare - the two men claiming the role of prime minister.
On Thursday Mr O'Neill told ABC News the country's defence chief, who was arrested in the mutiny, was now free.
He told the Australian broadcaster that Brig Gen Francis Agwi was no longer under house arrest and the group of soldiers had withdrawn from Murray Barracks to Toorama.
Mr O'Neill and Sir Michael have been wrangling over the leadership role for six months.
Before Mr O'Neill's latest reported announcement, the Australian government has released a statement saying it was ''very concerned'' and ''monitoring developments closely''.
''We want a PNG which is politically stable and economically successful,'' said the statement from Prime Minister Julia Gillard's office. ''The military has no place in PNG politics.''
''It is critical therefore that this situation be resolved peacefully as soon as possible, with the PNG Defence Force chain of command restored.''
'Within a week'
The leader of the soldiers, retired Colonel Yaura Sasa, is a former defence attache to Indonesia.
He declared himself commander after placing the head of the defence forces, Gen Francis Agwi, who backed Mr O'Neill, under house arrest early Thursday.
Col Sasa has denied staging a mutiny and said instead that he was appointed by Sir Michael's government.
''My task is restoring the integrity and respect of the constitution and the judiciary," he said at a press conference on Thursday at the military headquarters.
"I am now calling on the head of state to immediately implement Sir Michael's post as prime minister," he added.
If this was not done within a week, he said, he ''may be forced to take necessary actions''.
However shortly after his statement, Deputy Prime Minister Belden Namah said that 15 of the 30 or so men supporting Col Sasa had been arrested.
Mr Namah also told reporters that the soldiers did not have the wider support of the military.
According to the ABC report, the soldiers overpowered guards at Taurama barracks and took the commanding officer there captive.
They then moved to Murray Barracks, placing Brig Gen Agwi under house arrest.
On Thursday Mr O'Neill said the mutiny leader, Colonel Yaura Sasa, was being "dealt with".
The incident is the latest conflict in a power tussle between the two men claiming the South Pacific nation's top job.
Sir Michael left Papua New Guinea in March to receive treatment for a heart condition. In June, his family announced he was standing down from politics, a move he later said had been taken without consulting him.
He remained out of the country for five months and in August, MPs declared the position vacant and that Sir Michael was no longer an MP. Mr O'Neill was elected by 70 votes to 24, replacing acting Prime Minister Sam Abal.
A Supreme Court ruling in December 2011 then stated that parliament had acted illegally by electing Mr O'Neill prime minister. The court also ruled in a 3-2 decision that Sir Michael should be ''restored to the office of prime minister''.
Mr O'Neill, who is backed by the civil service and effectively running the country, refused to step down.
Last week, Sir Michael showed up in parliament waving court documents and demanding to be reinstated. A rowdy scene ensued, with Sir Michael being ejected.
Mr O'Neill has not made any statement on the situation, the Associated Press news agency reported.
However, Australia's High Commissioner in Port Moresby, Ian Kemish, has spoken to Mr O'Neill, who told him that authorities were taking steps to manage the situation.
The Australian Department of Foreign Affairs and Trade has issued a travel advisory to ''limit travel around Port Moresby today''.
Australia's foreign minister, Kevin Rudd, has also spoken with Mr Kemish on the incident, a department spokesperson added.
''The overall situation in Port Moresby is calm, with business as usual,'' she said.