Rescue teams are searching for an airliner with at least 150 people on board that has gone missing over Brazil's Amazon rainforest.
Air force jets searched the densely forested region for wreckage.
Authorities are still trying to establish whether the Boeing 737-800 hit an executive jet reported to have made an emergency landing.
The Gol airlines flight left the Amazonian city of Manaus on Friday but failed to arrive in Brasilia.
The passenger plane had been due to arrive at 1812 local time (2112 GMT), before continuing to Rio de Janeiro's Antonio Jobim International Airport.
Brazil's civil aviation authority said the plane lost contact with air traffic control at 1650 local time (1950 GMT) around the remote, jungle town of Sao Felix do Araguaia.
"At this moment, we don't have any information that would indicate that the plane has either crashed or made an emergency landing," Jose Gomes Ferreira of Brazil's Aeronautical Command told Brazilian TV, the AFP news agency reported.
The BBC's correspondent Steve Kingstone in Sao Paulo said farm workers near the town of Matupa had reported a plane losing altitude and then the sound of an explosion.
The air force began searching close to the plane's last known position but when night fell the search was called off. It resumed at daylight.
The mayor of a remote Amazon town in the central state of Mato Grosso told Reuters news agency he believed the airliner had crashed on a farm in the area.
"Hospitals in the region are ready to receive the injured," he said.
Globo news agency says the airliner collided with a smaller, Brazilian-made Legacy aircraft which suffered wing damage before landing in the Cachimbo mountain range in Para state.
"It is not possible to state that there is a link between the incident that forced the crew (of the Legacy) to make an emergency landing at the Cachimbo airport and the disappearance of the GOL aircraft," the National Civil Aviation Agency (ANAC) said.
BRAZIL'S GOL AIRLINE
The low-cost Brazilian airline was founded in 2001
It is modelled on the British budget airline Easyjet
It had a fleet of 55 aircraft prior to the accident
The airline had not suffered a crash before
Gol is expected to have more than 100 aircraft by the year 2012
The Estado news agency reported that the Legacy was piloted by a US citizen who had taken off from the Sao Jose dos Campos airport near Sao Paulo.
Ramon Bueno, an aviation official in Sao Paulo, told Estado that the likelihood of mid-air collision was "inexplicable."
He said that the two planes were "very modern and have anti-collision systems, which sound an alarm to alert the plane to any obstacle".
Manaus is the capital of Amazonas state in the heart of the Amazon rainforest some 1,700m (2700 km) north-west of Rio de Janeiro.
The lost plane was a Gol airlines Boeing 737
The flight between Manaus and Rio is popular with foreign tourists but there was no immediate word on the nationalities of those aboard.
Gol, a low-cost airline, has expanded rapidly in recent years to become Brazil's number two carrier.
The travel editor of the Independent newspaper, Simon Calder, told BBC News 24 the plane had only been in service with the airline for 19 days.
He said the Boeing 737 had a "fantastic" safety record and only about 50 accidents had been reported despite the aircraft being produced for 40 years.
This is the first major crash involving Gol, which was founded in 2001. Mr Calder said it was the first crash involving an aircraft from a budget airline in more than 10 years.
In a statement Gol confirmed the plane involved was new, having only come into service this month.