Australia has vowed to leave "no stone unturned" in its efforts to find a group of mining executives whose plane went missing in West Africa.
The plane, carrying nine passengers, disappeared on Saturday while flying from Cameroon to Republic of Congo.
One of Australia's richest men, Ken Talbot, is amongst the missing.
The group left the Cameroonian capital Yaounde to visit iron ore projects in Yangadou, a remote area of Congo.
Australian Prime Minister Kevin Rudd has pledged consular and diplomatic assistance to the search efforts.
"This is deeply concerning to all of us," he said.
"We will leave literally no stone unturned in our efforts to try and help what is a concerning set of developments for these families."
Australia's High Commissioner to Nigeria was travelling to Cameroon along with two other officials, Foreign Minister Stephen Smith said.
Cameroon's government has said that, aside from six Australians, two British, two French and one American were also onboard the aircraft.
Cameroon's military is leading a ground and air search over dense forest.
Aviation officials say they lost contact with the aircraft one hour after it left Yaounde.
The plane was chartered by Australian mining company Sundance Resources.
The area they were visiting includes the Mbalam project which straddles Cameroon and Congo - where Sundance Resources is hoping to extract 35m tonnes of iron ore a year.
Sundance Resources said after it was reported that the plane had not landed on Saturday its efforts had been focused on co-ordinating with the authorities in Cameroon, Congo and neighbouring Gabon.
Mr Talbot is a non-executive director of Sundance with an estimated wealth of $840 million (£567m), according to BRW business magazine's latest rich list.