Members of a United Nations-authorized force deployed to stabilize strife-torn Haiti are attempting to rescue four Salvadoran military personnel after gangs attacked their armored vehicle earlier this week.

The whereabouts of the Salvadoran team of aviation technicians who were part of the U.N.-backed Gang Suppression Force remain unknown. While security experts fear they might have been killed, Salvadoran and U.S. officials believe all four survived the attack.

Salvadoran President Nayib Bukele’s government is looking for ways to negotiate the release of the captured men before resorting to a harsher military response, said one person involved in the rescue efforts. Bukele, who gained global prominence with his iron-fisted crusade that put tens of thousands of Salvadoran gang members behind bars, said he was ready to confront and “finish off” the gangs.

Authorities have received no response to their overtures to Haitian gangs so far. The gangs have responded with heavy gunfire as security forces close in on their headquarters in one of Haiti’s largest security operations in recent years. No one has demanded a ransom for the men, said people familiar with the situation.

The four men were ambushed earlier this week by gang members as they returned to base after buying food in Croix-des-Bouquets, a town about 9 miles northeast of the Haitian capital of Port-au-Prince.

The Salvadoran armed forces said intense gang gunfire set the four men’s armored vehicle ablaze. Salvadoran forces deployed military helicopters, forcing the gang members to retreat as they fired heavily at the aircraft.

The rescue attempt was “hampered by the intensity of the fighting and security conditions in the area,” the Salvadoran military said in a statement.

The U.S. offered “the full support of the U.S. government,” said the U.S. ambassador to the U.N., Mike Waltz, on Monday. “America stands shoulder to shoulder with them.”The attack took place in an area controlled by two of the country’s most powerful gangs, said William O’Neill, the U.N.’s designated expert on human rights in Haiti, who recently returned from the country. The Chen Mechan, or “Bad Dog,” gang led the ambush with support from the 400 Mawozo, or “400 Country Bumpkins” gang. Gang members set fires so that the smoke would hamper helicopter rescue efforts, said O’Neill.

“Rescue attempts have faced heavy resistance,” he said, adding that it was unclear who held the Salvadorans or where. Bukele said the men were noncombatant aviation support staff.

In a defiant message addressed to the gang members in Haitian Creole, Bukele threatened further military action if they weren’t returned alive. “I want you, the gang leaders, to understand one thing very clearly: today’s attack is nothing compared to what is coming if we do not get all four hostages back alive,” Bukele said.

“We have already finished off the gangs in El Salvador, and we can finish you off as well,” he wrote on X Wednesday. A video posted by Bukele showed armored cars firing shots along an abandoned, garbage-strewn road as an armored bulldozer cleared a burning street barricade.

El Salvador is one of several countries that has contributed to the Gang Suppression Force, which is authorized to have up to 5,500 personnel. The force, led by a Mongolian general and comprising close to 2,000 soldiers from countries including Chad, Sri Lanka, Guatemala and Bangladesh, is the latest international effort to restore order in a country dominated by powerful, heavily armed gangs.

The force aims to aid Haiti’s police and armed forces in their efforts against the gangs, which have plunged Haiti into chaotic violence since the assassination of President Jovenel Moïse by Colombian mercenaries in 2021. The Haitian government has also turned for help to U.S. private security contractor Eric Prince, who is using a force of drone operators to fight the gangs.

“Haiti’s gangs have escalated their violence as they are increasingly willing and able to confront international forces directly,” said María Fernanda Arocha, an analyst for Acled, an independent organization based in Madison, Wisconsin, that collects and analyzes worldwide data on violence and conflict. “This attack demonstrates how gangs continue to adapt their tactics and undermine security efforts, further threatening Haiti’s already fragile political transition.”

More than 1,100 Haitians have been killed and some 400 wounded in gang-related violence between July and early October, according to the U.N. The gangs are believed to control up to 90% of the capital. Criminal violence has displaced some 1.5 million Haitians, pushing millions of residents to the brink of famine, the U.N. estimates.

Gangs have been stepping up reprisals against civilians and attacking rural areas, the U.N. said. In August, the powerful Viv Ansanm gang, which has been designated a foreign terrorist organization by the U.S., attacked the mountain town of Kenscoff southeast of Port-au-Prince, killing at least 47 people. The gang took another 52 residents hostage, of whom 13 were later freed, the U.N. said.

The violence has also jeopardized Haiti’s precarious return to democracy. Presidential and legislative elections scheduled for December have been postponed indefinitely. The country hasn’t held elections since 2016.