Roughly 1,000 soldiers from El Salvador have been sent to Haiti in a string of deployments over recent days, a security official in Haiti told Reuters on Tuesday. The move follows the disappearance of four Salvadoran members of the UN-backed force operating in the country.

It is not yet clear who will command the new Salvadoran contingent or how long the troops will stay in the Caribbean nation.

Haiti's government, which has gone without elected leaders for years, has struggled to gather foreign backing in its fight against armed gangs. Those groups have seized control of much of the capital and surrounding regions, and the resulting violence has displaced nearly 1.5 million people.

Tensions have climbed since Salvadoran President Nayib Bukele announced a military operation to recover the missing men. They have been unaccounted for since they jumped from a burning vehicle during a gang ambush on the outskirts of Port-au-Prince on Oct. 18. Bukele said the four were air force technicians returning to base from a market, and that he believed criminal groups were holding them hostage.

No Haitian gang leader has claimed responsibility for the attack. Wilson Joseph, leader of the 400 Mawozo gang, has released videos in which he denies holding the Salvadorans hostage and rejects responsibility for what happened to them.

Before the ambush, El Salvador had deployed at least 124 troops as part of the UN-backed Gang Suppression Force, according to a UN report. Bukele later said those personnel were pilots and technicians sent to evacuate injured soldiers, not to fight. He has not said how many soldiers he intends to send in total, and his office did not respond to requests for comment.

Logistics for the deployment have drawn on outside help. A U.S. military official told Reuters that U.S. Southern Command supplied C-130 military transport jets to partly assist in bringing the Salvadoran troops to Haiti.

An Ecuadorean commercial airline, Aeroregional, had also scheduled further direct flights to Haiti on Tuesday, according to Flightradar24 data. There are usually no direct flights between the two countries. The airline did not immediately respond to a request for comment.

On Monday, President Luis Abinader of the Dominican Republic, the only country that shares a land border with Haiti, said at a news conference that he was prepared to offer logistical support to the arriving Salvadoran forces.