Shas chairman Eli Yishai sent a letter to Prime Minister Benjamin Netanyahu over the weekend, in which he asked the PM to push for the release of convicted spy Jonathan Pollard during his visit to the United States.

Pollard, a former civilian intelligence analyst, was sentenced to life in prison in 1987 on charges of spying on the U.S. for Israel.

"The demand for Pollard's release is justified in the days in which we are asked to make concessions for peace," wrote Yishai to Netanyahu, who leaves for the U.S. on Saturday night.

Yishai mentioned in his letter that in the past, Netanyahu recognized the need to act for Pollard's release, being the first prime minister to discuss the issue with the U.S. government.

He also stressed that Pollard has been in prison for 25 years now, and that his medical condition is unstable.

Yishai added that former U.S. officials involved in Pollard's case admit that his sentence was too severe and they too believe Pollard should be released.

"The United States is a close ally so I am positive they will heed the request," Yishai wrote.

Several MKs joined Yishai's plea and gave Netanyahu a letter on the subject addressed to U.S. President Barack Obama.

Netanyahu, however, has no plans of taking the letter with him to the United States.

"The Prime Minister has brought up Pollard's case several times in discussions with the U.S. government, so he does not need to do so through a letter. The proper procedure for transferring letters is to send them in an official manner to the U.S. embassy in Israel," the Prime Minister's Office said in a statement.