The sources, who spoke on condition of anonymity, said the Colombians' experience fighting leftist guerrillas and drug traffickers in their home country made them attractive recruits for the UAE, whose relatively inexperienced army is part of an Arab coalition helping Yemen's government fight a war against Huthi rebels.
"Colombian soldiers are highly prized for their training in fighting guerrillas," one source, a Colombian former army officer, told AFP in Bogota.
"Colombians have so many years of experience in war that they can take it."
The presence of Colombian troops in Yemen's bloody conflict further complicates what is already a messy proxy war pitting Iran, which backs the rebels, against a US-backed Arab coalition led by rival regional heavyweight Saudi Arabia.
Since the rebels began seizing large swaths of territory in July 2014, the conflict has left 6,000 dead and 28,000 wounded, many of them civilians, according to the United Nations.
Colombian soldiers are frequently recruited by international private security firms for jobs in places like Iraq, Afghanistan and Sudan.
The source, a 48-year-old man who left the army in the late 1990s, was himself formerly employed by Blackwater, the controversial US company now known as Academi that was contracted by the Pentagon to provide military and security services in Iraq.
He was hired in 2004, amid what he called a "boom in the recruitment of Colombians to fight in Iraq," and has since worked in Afghanistan, the UAE, Qatar and Djibouti.
Latin Americans were popular with firms like Blackwater, he said: in all, 1,500 Colombians, 1,000 Peruvians, 500 Chileans and 250 Salvadorans were contracted in Iraq between 2004 and 2006.
He said that from around 2010 the UAE began recruiting Colombians for a private army it was forming at a base in the middle of the desert called Zayed Military City.
Prized as special forces commanders or Blackhawk helicopter pilots, the Colombians are paid around $3,300 a month -- five times less than equivalent American contractors, but a small fortune by Colombian standards, he said.
"They were not recruited for combat missions. It was for security and protection missions. So they are not considered mercenaries," he said.
- Extra $120 a day -
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But a month ago about 300 of the 3,000 Colombians recruited so far by the UAE "decided voluntarily" to go fight as full-fledged mercenaries in southern Yemen, based in the port of Aden, the source said.
Their deployment came after 30 Emirati soldiers were killed in Yemen in a missile attack blamed on the rebels.
The source said the UAE initially planned to send 800 Colombians but met resistance from the recruits, who complained that fighting in Yemen was beyond the scope of their original contracts.
"The Colombians were supposed to pass unnoticed as local Emirati troops, and that caused a large number of them to desert," the source said. "They said their contract was in the UAE and not fighting other people's wars."
He said the UAE sought to sweeten the deal by limiting tours of duty to three months and offering an extra $120 a day in combat pay.
"They want to make war an industry using Colombians as cannon fodder," he said.
No Colombians have been killed in Yemen so far, he added, denying recent reports from rebel-linked media outlets of Colombians killed in combat.
"There are wartime interests behind that fear campaign," he said.
Another Colombian source, security expert John Marulanda, said lucrative salary offers from foreign firms are a "drain" on Colombia's well-trained but poorly paid military.
"Around 2011, very well-trained people started leaving," Marulanda told AFP.
"The UAE is participating in the coalition by discreetly sending mercenaries to Yemen. And it is a fact that among those mercenaries there are former members of the Colombian military."
A retired senior Colombian officer who asked not to be named said this had "created quite a lot of problems" for the Colombian defense ministry.
"The best officers are going" to the Emirates, the retired officer told AFP. "They are not recruiting their own nationals. They prefer to recruit people who are already prepared and specialized."
Colombia's army has spent the past five decades fighting a low-intensity conflict against leftist rebel groups.
The conflict, which has also drawn in right-wing death squads and drug traffickers, has killed more than 220,000 people.