By Laura Zuckerman

(Reuters) - An Iranian-American pastor imprisoned in Iran for more than a year because of his Christian faith is in declining health for lack of proper nutrition and necessary medication, a group seeking his release said on Tuesday.

An Iranian court in January sentenced Saeed Abedini, 33, a naturalized U.S. citizen, to eight years in prison for undermining national security by working to establish home-based Christian churches in Iran from 2000 to 2005.

The Washington-based American Center for Law and Justice, which has petitioned Congress and President Barack Obama to seek Abedini's release, said on Tuesday that the pastor's father was allowed to visit him on Monday for the second time since he was moved a month ago to a prison that houses violent offenders.

"It's a worsening situation," said Gene Kapp, spokesman for the center. "His father reported that he has visibly lost weight, he is covered from head to toe with lice because of lack of basic hygiene and he has been refused medicine for internal injuries he suffered at the beginning of his incarceration."

The pastor's wife, Naghmeh Abedini, has said she feared for her husband's life in remarks made a day after Iran reached an interim deal with major world powers to accept restrictions on its nuclear program in exchange for limited relief from economic sanctions that have hobbled its economy and slashed its oil exports.

Naghmeh Abedini, 36, faulted the Obama administration for failing to secure her husband's release as a condition of the deal. The U.S. State Department has criticized Saeed Abedini's conviction and sentence and has called for him to be freed.

White House deputy national security adviser Ben Rhodes told CNN last week that Obama has raised the issue of detained Americans with Iranian leaders.

The Abedinis moved to Boise, Idaho, in 2005 after establishing home churches in Iran. Saeed Abedini traveled to Iran on those missions until 2009, when he was told by Iranian officials to restrict his work to non-religious, humanitarian projects.

The pastor was establishing a non-sectarian orphanage in Iran when he was detained last year. The American Center for Law and Justice's online petition demanding his return had gathered more than 71,000 signatures on Tuesday.

The U.S. State Department has criticized his conviction and sentencing, saying it was deeply concerned about the fairness and transparency of his trial, and has called for Abedini's release.

(Reporting by Laura Zuckerman in Salmon, Idaho; Editing by Alex Dobuzinskis and Gunna Dickson)