Haiti’s security forces have been locked in a fierce gun battle with assailants who assassinated President Jovenel Moise at his home, plunging the already impoverished, violence-racked nation deeper into chaos.

The police had killed four of the “mercenaries” and captured two more, Police General Director Leon Charles said in televised comments late on Wednesday (local time), adding security forces would not rest until they had all been dealt with.

“We blocked them en route as they left the scene of the crime,” he said. “Since then, we have been battling with them.

“They will be killed or apprehended.”

Mr Moise, a 53-year-old former businessman who took office in 2017, was shot dead and his wife, Martine Moise, was seriously wounded when heavily armed assassins stormed the couple’s home in the hills above Port-au-Prince about 1am (1500 AEST).

Haiti’s ambassador to the US, Bocchit Edmond, told Reuters in an interview the gunmen were masquerading as US Drug Enforcement Administration (DEA) agents as they entered Mr Moise’s guarded residence under cover of nightfall – a move that would likely have helped them gain entry.

The brazen assassination, which drew condemnation from Washington and neighbouring Latin American countries, came amid political unrest, a surge in gang violence, and a growing humanitarian crisis in the poorest nation in the Americas.

The government declared a two-week state of emergency to help it hunt down the assassins, whom Mr Edmond described as a group of “foreign mercenaries” and well-trained killers.

The gunmen spoke English and Spanish, said interim Prime Minister Claude Joseph, who assumed the leadership of the country, where the majority speak French or Haitian Creole.

“I am calling for calm. Everything is under control,” Mr Joseph said on television alongside Police General Director Charles.

“This barbaric act will not remain unpunished.”

The first lady had been flown to Florida for treatment where she was in a stable condition, Mr Joseph said.

Mr Moise was killed by gunmen masquerading as US agents.

Haiti, a country of about 11 million, has struggled to achieve stability since the fall of the Duvalier dynastic dictatorship in 1986, and has grappled with a series of coups and foreign interventions.

US President Joe Biden denounced the killing as “heinous” and called the situation in Haiti worrisome.

“We stand ready to assist as we continue to work for a safe and secure Haiti,” he said.

US Secretary of State Antony Blinken, in a call with Mr Joseph, expressed Washington’s commitment to work with Haiti’s government to support “democratic governance, peace, and security”, a spokesman said.

Many Haitians had wanted Mr Moise to leave office. Since he took over in 2017, he has faced calls to resign and mass protests – first over corruption allegations and his management of the economy, then his increasing grip on power.

Lately, he presided over a worsening state of gang violence that rights activists say is linked to politics and business leaders using armed groups for their own ends.

Mr Moise had talked of dark forces at play behind the unrest: Fellow politicians and corrupt oligarchs unhappy with his attempts to clean up government contracts and to reform Haitian politics. He provided no proof of this.

The streets of the usually bustling capital were mostly deserted on Wednesday and the airport was closed.

The UN Security Council expressed deep shock and sympathy at Mr Moise’s death before a closed-door meeting on Thursday.

A UN peacekeeping mission – meant to restore order after a rebellion toppled then-president Jean-Bertrand Aristide in 2004 – ended in 2019 with the country still in disarray.

Mr Moise’s murder comes amid a power vacuum. The banana exporter-turned-politician had ruled by decree for more than a year after the country failed to hold legislative elections.

Just this week he nominated a prime minister to replace Mr Joseph – who was meant to be only an interim PM – but the official has yet to be sworn in. The head of the Supreme Court of Justice died in June of COVID-19 amid a worrying surge in infections and has yet to be replaced.

An election is scheduled for September.

-AAP