Uruguay's President Jose Mujica has apologised for the alleged rape of an 18-year-old Haitian man by Uruguayan peacekeepers in the country.
Those responsible would receive the "harshest sanctions", Mr Mujica wrote to Haitian President Michel Martelly.
Five Uruguayan marines were accused after a video clip of the alleged abuse appeared on the internet.
The UN mission in Haiti (Minustah) and the Haitian authorities have also launched investigations.
The case has provoked widespread public anger in Haiti, and there have been protests outside the UN base.
"We apologise for the outrage that some soldiers from my country have perpetrated," Mr Mujica said in a letter to Mr Martelly.
"Although the damage is irreparable, be assured that we will fully investigate the matter and apply the harshest sanctions to those responsible."
Mr Martelly has condemned the alleged assault, saying it "revolts the conscience of the nation".
The alleged victim and his mother have told Haitian radio stations that he was raped by the Uruguayan marines in the UN base at Port Salut.
They have also given evidence to Haitian police and a local judge.
The UN mission in Haiti has said it is taking the allegations "very seriously".
UN deputy spokesman Eduardo del Buey told the Associated Press that the five alleged attackers had been confined to barracks pending the outcome of the three investigations.
"If the investigations prove that the allegations are true, we would expect that these people be prosecuted in the full extent of the law," he said.
Uruguay is one of the main contributors to the 12,000-strong UN force, which first arrived in Haiti in 2004.
The peacekeepers were brought in to restore order following the overthrow of President Jean-Bertrand Aristide, and its mandate was extended after the devastating earthquake in January 2010.
But Minustah has also drawn controversy, including allegations of excessive use of force.
President Martelly has acknowledged that Haiti still needs the peacekeepers, but wants their security role reduced and eventually replaced by a Haitian force.
Brazil, the lead nation in Minustah, has announced that it plans to start withdrawing troops.
But Brazilian Defence Minister Celso Amorim said the decision was not related to the latest allegations.