An aide to Nabih Berri indicated today that the Shiite Amal leader was not willing to mediate in the release of seven Americans kidnaped in Beirut during the past year, and the Islamic Jihad organization said they faced a "black fate" if the U.S. government harmed the Lebanese people.

Berri appeared to have hardened his line on the issue despite earlier pledges to do whatever he could to find the seven Americans. Yesterday, the chief of Amal's Politburo, Akef Haidar, had said the organization would seek to free the Americans if the United States made a specific request.

Berri also threatened to sue the United States in the International Court for its moves to isolate Beirut International Airport following the hostage crisis. He accused Washington of betraying a promise made prior to the ending of the 17-day hijack drama to safeguard Lebanon's sovereignty and security.

"What the U.S. has done is a betrayal of its commitments, an encouragement to violence and an encroachment on Lebanon's sovereignty and security," he said, adding that Lebanon was entitled to compensation.

A source close to Berri, who also is Lebanon's justice minister, said neither he nor his Amal militia was concerned with efforts to gain freedom for seven Americans kidnaped in Beirut during the past 16 months. The loose and undefined Islamic Jihad is claiming to hold the seven missing Americans.

"We are not mediators and we are not prepared to mediate," the source said.

A handwritten communique delivered to the privately owned News of Lebanon agency quoted Islamic Jihad as saying President Reagan "will shoulder the full responsibility" if his administration "attempts, directly or through Israel, any aggression against the oppressed in Lebanon. For the second time, we emphasize that the seven Americans with us will face a black fate if the American administration commits any foolhardiness against our people."

The same statement was telephoned by anonymous callers to two private radio stations in Beirut.

In a statement yesterday, callers claiming to speak on behalf of the same group said they would turn "the White House into a black house," in reaction to Reagan's orders for measures to shut down Beirut airport. It described the American moves as "economic terrorism" and vowed that the Lebanese people would retaliate with the "closure of all the region's airports to all U.S. planes."

Meanwhile, ambassadors of the 17 Arab League countries with embassies in Washington are expected to meet with State Department officials Friday to protest the administration's campaign to force closure of Beirut International Airport, Lebanese Ambassador Abdallah Bouhabib said Wednesday.

Bouhabib and other Arab diplomatic sources said that at the meeting with Richard W. Murphy, assistant secretary of state for Mideast affairs, the ambassadors will argue that the U.S. action would punish the Lebanese government and people unjustly and not deter hijacking and terrorism in the Middle East.

The U.S. action was announced Monday after the freeing of the 39 American hostages.