The longest-held American prisoner in Iran began what he said would be a weeklong hunger strike on Monday to demand his immediate release, appealing directly to President Joe Biden to negotiate freedom for him and other prisoners with dual citizenship.

Siamak Namazi, an Iranian American businessperson, made his appeal in a letter written from Iran’s notorious Evin Prison in the capital, Tehran, where he has been held for more than seven years.

The Biden administration has emphasized that bringing home Americans unjustly held abroad is a top priority that it considers “every day.” But U.S. leverage to press Iran into an agreement has been limited as relations between the two countries have grown more strained.

In an update posted to Twitter on Tuesday, Namazi’s lawyer, Jared Genser, said that his client decided to keep up his strike after prison officials called a meeting to try to persuade him to end it Tuesday.