PORT-AU-PRINCE, July 8 (AFP) - Four "mercenaries" were killed and two taken into custody after the assassination of Haitian President Jovenel Moise, police said on Wednesday (Jul 7), adding that officers were conducting an operation in the capital Port-au-Prince.

"Four mercenaries were killed, two were intercepted under our control. Three policemen who had been taken hostage have been recovered," said the director general of Haiti's national police Leon Charles in a televised statement.

He said that police pursued the assassins immediately after the gun attack on Moise and his wife, who survived, at their home in Port-au-Prince early on Wednesday.

A government minister had said earlier that Moise's "alleged assassins" were in custody, less than 24 hours after the leader of the impoverished Caribbean nation was killed.

"Alleged assassins of (Moise) intercepted by the National Police in Pelerin shortly before 6pm tonight," deputy communications minister Frantz Exantus tweeted, adding that more details would be released shortly.

The attack at Moise's private residence, which also left First Lady Martine Moise wounded, pitched the crisis-hit country into uncertainty, leaving citizens fearful as shocked world leaders called for calm.

The United Nations Security Council will hold an emergency meeting on Haiti at around midday on Thursday, and interim Prime Minister Claude Joseph - who is due to be replaced this week - declared a "national state of siege" granting the executive additional powers, and said he was now in charge of the country.

As international outrage spread over the killing, the airport was closed in Port-au-Prince, but witnesses said the city was quiet with the streets deserted and no extra security forces on patrol.

The attack took place around 1am. Joseph said the president was "assassinated at his home by foreigners who spoke English and Spanish".

"This death will not go unpunished," Joseph said in an address to the nation.

Moise's wife Martine was first treated in a local hospital before being evacuated by air ambulance to the Ryder Trauma Center in Miami.

Earlier, Haiti's ambassador to Washington, Bocchit Edmond, told reporters the killers were "professional" mercenaries who disguised themselves as US Drug Enforcement Administration agents and may have already left the country.

"We have a video and we believe that those are mercenaries," he added.

Moise had ruled Haiti, the poorest country in the Americas, by decree after legislative elections due in 2018 were delayed in the wake of disputes, including on when his own term ends.

In addition to the political chaos, kidnappings for ransom have surged in recent months, reflecting the growing influence of armed gangs in the country.

Haiti also faces chronic poverty and recurrent natural disasters.