Former U.S. Assistant Secretary of Defense Lawrence Korb said on Tuesday that the U.S. should release convicted Israeli spy Jonathan Pollard from prison, saying Pollard's punishment was "disproportionate."

"As an American, I can't accept the injustice of it," Korb said.

Korb was assistant secretary of defense when Pollard was arrested by U.S. authorities in 1985. Pollard was later convicted of spying for Israel and sentenced to life in prison.

Korb, now a senior fellow at the Center for American Progress think tank in Washington, is currently visiting Israel. He met with Prime Minister Benjamin Netanyahu and President Shimon Peres earlier this week.

"Pollard is the only person in U.S. history to be served a life sentence for spying for a friendly country," Korb said. "Israel is undoubtedly a friend, not an enemy. The average sentence for spying for a friendly country is only a few years. He needs to be released not because of interests, but because it is the right thing to do."

"It is time for the Israeli public to make its voice heard," Korb said. "Precisely because we are such close friends, it is important that President [Barack] Obama hears your voice."

More than 100,000 Israelis have signed a petition calling for Pollard's release. The petition will be presented to Obama during his visit to Israel later this month. Prominent signers of the petition include former President Yitzhak Navon, writers A.B. Yehoshua and David Grossman and Nobel Prize winners Dan Shechtman, Aaron Ciechanover and Robert Aumann.