Setting an emergency meeting over the crisis, the UN Security Council. members unanimously called "for the perpetrators of this abhorrent crime to be swiftly brought to justice," and for "all parties to remain calm, exercise restraint."
Haitian police have killed four "mercenaries" who they said were behind the assassination of President Jovenel Moise and took two more into custody, as the impoverished and crisis-hit Caribbean nation was pitched into uncertainty.
Police did not identify the suspects or say what their motives were for the gun attack on Moise and his wife Martine, who survived, at their private residence in the capital Port-au-Prince early on Wednesday.
Police chief Leon Charles said there were still more members of the hit squad at large.
"As I speak, the police are engaged in battle with these assailants," he said on late Wednesday. "We are chasing them so that either in the exchange of fire they will be killed or we will apprehend them."
Interim Prime Minister Claude Joseph declared a national "state of siege" and said he was now in charge.
At the UN Security Council, members unanimously called "for the perpetrators of this abhorrent crime to be swiftly brought to justice," and for "all parties to remain calm, exercise restraint" and avoid "any act that could contribute to further instability." An emergency meeting on the crisis has been set for midday Thursday.
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.
"Four mercenaries were killed, two were intercepted under our control. Three policemen who had been taken hostage have been recovered," said Charles, the head of Haiti's national police.
The attack took place around 1:00 am (0500 GMT) at Moise's home. Shell casings could be seen on the street outside as forensics experts combed the scene for evidence. A nearby car was peppered with bullet holes.
Magistrate Carl Henry Destin told the Nouvelliste newspaper that the president's body had twelve bullet holes in it, from large caliber rifles and smaller 9mm weapons, to the forehead, chest, hips and abdomen.
"The president's office and bedroom were ransacked. We found him lying on his back, blue pants, a white shirt smeared with blood, his mouth open, his left eye gouged out," he said.
Moise's wife was first treated at a local hospital then rushed by air ambulance to the Ryder Trauma Center in Miami.
Joseph said she was "out of danger", later adding that "her situation is stable."
Their daughter Jomarlie was in the home during the attack but hid in a bedroom, Destin, the magistrate, said.
He said a maid and another domestic staff member had been tied up by the commandos who allegedly shouted "DEA operation" as they burst in.
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.
Haiti's ambassador to Washington, Bocchit Edmond, also said the killers were "professional" mercenaries disguised as US Drug Enforcement Administration agents.
The unpopular Moise had ruled Haiti, the poorest country in the Americas, by decree after legislative elections due in 2018 were delayed.
In addition to the political chaos, kidnappings for ransom have surged in recent months.
The capital's streets were at a standstill in the hours after the assassination, with just a handful of citizens outdoors.
"We didn't expect it. This is another earthquake in Haiti," said a mother of two who gave her name only as Bernadette, referring to deadly 2010 quake.
"I can't believe it, I can't believe it," said 50-year-old Jacquelyn.
Haiti will observe two weeks of national mourning from Thursday.
Joseph – who spoke by telephone to US Secretary of State Antony Blinken Wednesday – has only been in his post for three months, and was due to step down within days after Moise named his replacement on Monday.
As well as presidential, legislative and local elections, Haiti was due to hold a constitutional referendum in September after it was twice postponed due to the coronavirus pandemic.
US President Joe Biden condemned the killing as "horrific" and said Washington was ready to assist in any way.
Washington also called for Haiti to proceed with the elections, with the State Department spokesman saying a fair vote would "facilitate a peaceful transfer of power to a newly elected president."
UN Secretary-General Antonio Guterres called on Haitians to "remain united" and "reject all violence."
European Union foreign policy chief Josep Borrell warned of "risk of instability and a spiral of violence."
READ MORE: Haiti protesters call on president to resign
Governed by decree
Moise, a successful businessman, burst onto the political stage in 2017 and campaigned as a populist. He was sworn in in February 2017.
The end date of his mandate however became the source of a standoff, as Moise maintained that his term of office ran until February 7, 2022, but others said it ended on February 7, 2021.
The disagreement is because Moise was elected in a 2015 vote that was cancelled for fraud, and then re-elected in November 2016.
Without a parliament, the country fell further into crisis in 2020.
Many feared Haiti could tip further into violence.
"How much worse can hell get?" asked Haiti expert Irwin Stotzky at the University of Miami.
"Haiti faces even more violence and death and failure as a democratic nation than ever before, which is hard to imagine given its recent and chaotic history."
The killing comes days after Moise appointed Ariel Henry, a French-trained neurosurgeon, as Haiti's new prime minister.
Henry, 71, is close to the opposition, but his appointment was not welcomed by the majority of opposition parties.