Heavy gunfire has broken out in Haiti's capital near the presidential palace, where US Secretary of State Colin Powell was holding talks at the time.
It is Powell's second visit to Haiti in eight months
The government blamed supporters of the former President, Jean Bertrand Aristide, for the shooting.
UN forces returned fire. There were no immediate reports of any injuries.
Mr Powell was on a one-day visit to help promote political stability. He was due to meet young Haitians living with HIV, to mark World Aids Day.
The secretary of state was holding talks with caretaker Prime Minister Gerard Latortue and other key figures during Wednesday's shooting.
The BBC's Hannah Hennessy in Port-au-Prince said the gunfire came in three or four bursts over a 15-minute period.
The shots are believed to have been fired from a car passing through the suburb of Bel Air, two blocks from the presidential palace.
The area is known to be a stronghold of supporters of Mr Aristide.
UN peacekeeping forces returned fire and UN tanks were also brought in to protect the palace.
"They have to forcefully take on these armed individuals of the kind who were firing this morning," Mr Powell said after his talks at the presidential palace.
The UN troops stationed in Haiti have now been placed on alert.
UN peacekeepers were sent in after Mr Aristide was forced out of office by an uprising in February.
In recent months, renewed violence between government forces and supporters of Mr Aristide has claimed dozens of lives.
It is Mr Powell's second visit to Haiti this year, following a previous one-day visit in April.
On that occasion, he became the first senior US official to visit since the three-week rebellion that ousted Mr Aristide in February.
Mr Aristide has repeatedly accused the US and France of forcing him into exile.
But the US and France insist that Mr Aristide agreed to leave the country voluntarily, and signed a letter of resignation.
Since then, Haiti has suffered further turmoil. In October, Tropical Storm Jeanne left more than 2,000 dead and many more homeless.
Mr Powell announced last month that he would soon step down as US Secretary of State.