PORT-AU-PRINCE - Haitian authorities said on Friday they believe 140,000 people were killed in the earthquake that devastated the Caribbean nation and said gangs of robbers were preying on survivors desperately awaiting aid.

We are cleaning the streets of the dead bodies and putting them in mass graves. We have buried 40,000 people. We think there are 100,000 more on top of that, Aramick Louis, secretary of state for public safety, told Reuters. There are a lot of people under the rubble.

He said the main concern of the president and prime minister, who were coordinating the Haitian government's response from the judicial police headquarters near the airport, was that desperation was turning to violence.

We are sending our police into areas where bandits are starting to operate. Some people are robbing, are stealing. That is wrong, Louis said.

The people in the refugee places, once they do not find food and assistance, they are getting angry and upset. Our message to everyone is to stay calm.

Some looting broke out in downtown Port-au-Prince, with a body burned and at least two shots fired, a witness said.

Three days after the earthquake, governments across the world were pouring relief supplies and medical teams into the quake-hit Caribbean state -- already the poorest in the Western Hemisphere.

But huge logistical hurdles, including a clogged airport and badly damaged port, and the sheer scale of the destruction meant aid was not yet reaching hundreds of thousands of victims.

(Additional reporting by Tom Brown, Kena Betancur and Carlos Barria in Port-au-Prince, Stephanie Nebehay in Geneva, Steve Holland, Alister Bull and Phil Stewart in Washington; writing by Jane Sutton, Anthony Boadle and Pascal Fletcher; editing by David Storey)