President Reagan has called Syrian President Hafez Assad to ask for help in gaining release of seven Americans held hostage in Lebanon by Shiite extremists, the Beirut media reported, and the White House confirmed yesterday.

Assad reportedly promised Reagan that he would do what he could and said he has contacted the kidnapers in an effort to persuade them to release the hostages.

"They have turned down our good offices," Assad was quoted as telling Moslem clergymen.

A White House official said Reagan had telephoned Assad "about using Syrian influence to get their release" but added, "I don't think it was very recent."

He said this is the sort of effort in which Reagan was involved on behalf of the hostages but made a point of not discussing for fear publicity would prove harmful.

Assad was quoted as saying that it is difficult to deal with the Shiite extremists involved in the kidnaping but that Syrian officials were continuing to try to persuade them to release the Americans.

Assad was also quoted as telling Reagan that Syria made a distinction between "humanitarian matters and politics" and that, although he believes that "America is responsible for all our predicaments," there should be a "code of honor between the combatants" against kidnapings.