Australian commandos have landed in East Timor in an effort to quell three days of increasing violence.
Australian troops were warned to expect a dangerous mission
About 150 troops secured the airport in the capital, Dili, amid reports of mounting casualties in the city.
A further 1,300 Australians, and troops from Malaysia, New Zealand and Portugal are also heading to East Timor.
Reports say a number of people have died and many have been injured in Dili, where disgruntled ex-soldiers are fighting government troops.
Australia's ABC radio reported that a fleet of ambulances were requested after a battle outside Dili's main police station.
A UN official in the city described "constant gunfire" around the organisation's compound, and said the army was "on the rampage".
"Nobody knows actually what's going to happen, because the law and order seems to be totally collapsed," Jawad Omar told ABC.
A South Korean national was shot in the neck during the fighting and is recovering in hospital.
East Timor's Foreign Minister Jose Ramos Horta admitted on Wednesday that his government "could not control the situation".
Australia has experience of providing military aid to East Timor, as it led a UN-military force into the country in 1999 to end the unrest sparked when the population voted for independence from Indonesia.
Prime Minister John Howard sped up the deployment of the main Australian force to East Timor after hearing of the worsening security situation in the island nation.
He warned Australian MPs that the situation in East Timor was violent and volatile.
"This is a dangerous mission and a dangerous situation and we must not walk away from the possibility that casualties could be suffered by the forces that will go to East Timor," he told parliament.
The unrest began in March, when nearly 600 of the army's original force of 1,400 went on strike for better working conditions. They were subsequently sacked.
The violence escalated and five people were killed in clashes in April, and tens of thousands fled Dili fearing further unrest.
This week, the tension has again risen. At least three people have been killed since Tuesday, and gunfights around Dili are continuing for a third day, with shoot-outs reported close to the presidential palace and on the sea-front.
"These Australian forces will take immediate action to secure the perimeter of Dili International Airport," Mr Howard said.
"It's our expectation that this will ensure the airport remains open and functioning normally."
Australia is just one of the countries that have responded to East Timor's request for help in stabilising the situation.
In Portugal - the former colonial ruler of East Timor - Prime Minister Jose Socrates said Lisbon would send a contingent of paramilitary police in "a gesture of solidarity".
New Zealand Prime Minister Helen Clark said a detachment of troops was ready to deploy, but she first wanted more details of the mission.
"It's very important not to walk into what is a factional dispute, in some respects, and be seen to be taking sides," Ms Clark told New Zealand radio.