Later US officials said Bout had been arrested in a sting operation in which US anti-drugs agents posed as Farc fighters.
An investigation had been underway over the past eight to 12 months and "the undercover operation since November 2007", an official was quoted by the AFP news agency as saying.
Bout has run a network of air cargo companies in the Middle East, Africa, Eastern Europe and the US.
According to the UN and the US Treasury Department, he has sold or brokered arms that have helped fuel wars in Afghanistan, Angola, the Democratic Republic of Congo, Liberia, Rwanda, Sierra Leone and Sudan.
|"After the end of the Cold War there are a |
lot of extra weapons, especially of Soviet-Russian origin, available"
Pavel Felgenhauer, defence analyst
A 2005 report by Amnesty International, a Britain-based human rights organisation, said that Bout was "the most prominent foreign businessman" involved in trafficking arms to UN-embargoed destinations from Bulgaria, Slovakia, Ukraine, Kyrgyzstan and other countries.
His life was the inspiration for the Hollywood film Lord of War.
Bout has repeatedly denied the allegations.
Russia is likely to seek Bout's extradition, the RIA Novosti news agency on Thursday quoted a Russian law-enforcement official as saying.
"Russia is currently waiting for official confirmation from Thailand of the businessman's arrest. We should get that in two or three days. After that we can demand his extradition to Russia," an unnamed official was quoted as saying.
Pavel Felgenhauer, a Moscow-based defence analyst, told Al Jazeera that the international arms trade was worth billions of dollars.
"After the end of the Cold War there are a lot of extra weapons, especially of Soviet-Russian origin, available and a lot of skilled mercenary fighters including pilots and mechanics," he said.
"So they [arms dealers] deliver not just simply small arms or just guns, but MiG fighters. They deliver helicopter gunships, together with mercenary fighters, which have been used in wars in the Balkans, in Africa, in Asia.
"Really, it's like delivering pizza - it's really hot. This is a cash market. If you pay, you get your delivery in several days, on the spot together with mercenaries and you can go to war."
Felgenhauer said Bout "most likely knows a lot" about the trade.
"He can embarrass a lot of people - in Russia, but not only in Russia," he said.
Although Bout has been investigated by police in several countries, he has never been prosecuted for arms dealing.