WASHINGTON (AP) — The U.S. says North Korea is using detained American citizens as political pawns, after a 24-year-old Californian man was sentenced to six years of hard labor there.

Matthew Miller, of Bakersfield, was convicted Sunday of entering the country illegally to commit espionage. The court said he tore up his visa on arriving in Pyongyang (pyuhng-yahng) April 10 and had wanted to experience prison life so that he could secretly investigate North Korea's human rights situation.

State Department spokeswoman Marie Harf told reporters Monday that the U.S. disagreed with the sentencing. She said Miller and two other Americans should be released and returned home.

Harf said that despite North Korea's official claims to the contrary, it's increasingly clear that nation "seeks to use these U.S. citizens as pawns to pursue its own political agenda."