The Reagan administration has warned Iran of unspecified serious consequences in case of the death of U.S. hostages seized by a pro-Iranian group in Lebanon, administration sources said yesterday.

Secretary of State George P. Shultz sent the message, officials said, after a Beirut news bureau received a telephone call in mid-February saying that an American captive had been sentenced to death. The caller claimed to represent the pro-Iranian Islamic Jihad movement, thought to be holding at least four Americans.

No details of the possible consequences were detailed in the message, and officials refused to specify yesterday what actions the administration has in mind.

The link between the Iranian government and pro-Iranian terrorists in Lebanon is shadowy.

But, in a speech here March 25, national security affairs adviser Robert C. McFarlane volunteered that "there is . . . sufficient evidence that radical Shia terrorists are responsive to Iranian guidance for us to hold Tehran responsible for such terrorist attacks against United States' citizens, property or interests."

The administration served notice in a State Department statement yesterday that Libyan leader Muammar Qaddafi will be held "fully responsible" for Libyan terrorism against Americans or U.S. interests.

The statement was issued in response to a Libyan news agency report quoting Qaddafi as advocating that "suicide missions" and "revolutionary violence" be undertaken in the Middle East. Qaddafi reportedly made the comments Sunday night to a newly organized group, National Command for the Arab Revolutionary Forces.