Barack Obama has become the first sitting US president to visit a federal prison, amid a push to reform America's expensive and overcrowded correctional system.
Obama on Thursday (local time) toured the "B" block of El Reno Federal Correctional Institution in Oklahoma, where he also met six inmates convicted of drug offences.
Nearly a quarter of the world's prison population is concentrated in American jails. However, the United States accounts for less than five percent of the world's population.
Obama wants to cut the number of people incarcerated, curb use of solitary confinement and end mandatory minimum sentences.
"We have to consider whether this is the smartest way for us to both control crime and rehabilitate individuals," he said during his visit.
"We have to reconsider whether 20 year, 30 year, life sentences for nonviolent crimes is the best way for us to solve these problems."
After viewing a small cell that can hold up to three inmates, Obama said: "This is an outstanding institution within the system, and yet they've got enormous overcrowding issues."
The United States jails as many people as the top 35 European nations combined.
Black and Latino Americans represent 60 percent of the prison population while around 30 percent of prisoners are white.
Around 71,000 minors are also incarcerated in the United States.
"I think we have a tendency sometimes to almost take for granted, or think it's normal, that so many young people end up in our criminal justice system," Obama said. "It's not normal. It's not what happens in other countries."
The US prison system has also seen an explosion in costs. At US$80 billion, the budget for prisons represents a third of the Department of Justice's annual spending.