A U.S. permanent resident imprisoned in Iran on espionage charges is in extremely poor health after beginning an open-ended hunger strike to protest his detainment, his American lawyer said Wednesday.

Nizar Zakka, a Lebanese citizen who has been detained in Tehran's Evin prison since the fall of 2015, commenced his fifth hunger strike nine days ago. His blood pressure has already dropped to dangerously low levels, "complicating existing physical problems that are not being addressed," according to his lawyer, Jason Poblete.

Zakka has previously been denied all medical and consular services, though it is unclear whether that remains the case. Iranian prison guards have moved Zakka to solitary confinement as punishment for the strike.

"As he has done throughout this ordeal, Nizar maintains his innocence," Poblete said in a statement Wednesday. "The Iranian government should put politics aside and release Nizar on a humanitarian basis before this matter escalates, as did the recent case of an American in North Korea."

Iranian authorities arrested Zakka in September 2015 after he travelled to Tehran to attend a conference at the invitation of the Iranian government. Zakka, an advocate for Internet freedom, was sentenced to 10 years in prison on accusations that he was spying for the United States

Zakka is one of four permanent residents or U.S. citizens currently jailed in Iran.

Republican lawmakers have heaped blame on the Obama administration for offering Tehran an incentive to take American hostages after it paid $400 million in cash to the Iranian government in a calculated exchange that secured the release of four U.S. hostages. The United States has a longstanding policy that forbids government payments in exchange for hostages.

A senior Trump administration official told the Washington Free Beacon last month that the president was determined to bring all American hostages home during his tenure.

Over the past three months, Trump secured the release of two U.S. hostages: Aya Hijazi, an Egyptian-American aid worker imprisoned in Cairo for three years; and Otto Warmbier, a University of Virginia student who was detained in North Korea for a year and a half.

Warmbier died just days after he returned to the United States in a coma.