International arms smuggler Viktor Bout's sentencing was delayed Wednesday by a US judge for a second time, court papers showed.

Bout, convicted in November in New York federal court of conspiring to sell a huge arsenal to anti-American guerrillas in Colombia, will now be sentenced April 5, instead of March 28, Judge Shira Scheindlin ordered.

Scheindlin cited "the serious and complex nature of the issues" presented by defense attorneys and prosecutors ahead of the sentencing, which could see the Russian, popularly dubbed "the merchant of death," put behind bars for life. The minimum sentence is 25 years.

The sentencing had already been moved from March 12 following a defense request for more time to prepare arguments for leniency.

Bout was arrested in Thailand in 2008 at the end of a US sting operation that stretched from the Caribbean island of Curacao to former communist central Europe and a hotel in Bangkok.

US agents posing as members of Colombia's FARC, a US-designated terrorist organization, lured Bout from his native Russia, where he was under government protection, to conduct a deal for infantry and anti-aircraft weapons.

The mustachioed Russian is widely believed to have been the world's biggest black market arms dealer in the post-Cold War period, specializing in arming African warlords and dictators. He says he worked exclusively as a private air transporter -- although sometimes carrying legal shipments of arms.

Scheindlin ruled in favor of a February appeal by Bout's lawyers to let him out of solitary confinement for the first time in 15 months.