WASHINGTON — For days, members of the Obama administration have expressed outrage and threatened consequences for Libya’s welcoming celebration of the man convicted in the 1988 Lockerbie airline bombing that killed 189 Americans.

But United States officials conceded Monday that there was little they could do and that there remained the potential for an awkward encounter between President Obama and the Libyan leader, Col. Muammar el-Qaddafi, at the United Nations General Assembly in September.

“It’s not like we can send in the 82nd Airborne,” said a United States official who, like others in this article, requested anonymity to speak about the limited options available to the Obama administration.

Administration officials said the Libyan welcome last week of Abdel Basset Ali al-Megrahi, the only person convicted in the Pan Am jet bombing over Lockerbie, Scotland, that killed 270 people, sharply strained the already uneasy American relationship with Libya. Yet the practical implications of the strain were unclear.

Ian C. Kelly, the State Department spokesman, told reporters on Monday that the United States was “going to be watching very closely how they receive this man and, if they continue to lionize him in a public fashion, that these kinds of public demonstrations can only have a profoundly negative effect on our relationship.”

Pressed on what he meant by a “profoundly negative effect” — and whether Washington was considering sanctions against Libya — Mr. Kelly replied that “it’s premature for me to say that we’re actually sitting down and considering concrete actions that we would take.”

A senior administration official characterized the Libyan welcome of the bomber as a setback in relations between the United States and Libya, but said that it was too early to determine how the Obama administration would respond beyond angry words. The official also noted that it was August and that much of the administration, including the president, was not in town.

The United States government lifted economic sanctions against Libya and restored diplomatic ties after Libya gave up its nuclear and chemical weapons program under the Bush administration in late 2003. An American Embassy was opened in Tripoli, and former Secretary of State Condoleezza Rice visited in 2008, the first time the nation’s top diplomat had been to the Libyan capital since a visit by John Foster Dulles in 1953.

But Libyan officials have said recently that they are dissatisfied that the United States has not done more in return for Libyan concessions, like providing civilian nuclear technology and some conventional weapons systems. Libyans have also been upset about a State Department report that strongly criticized the country’s record on human rights.

Obama administration officials have responded that the Libyans are unrealistic in their expectations of how quickly the relationship will improve.

Mr. Megrahi, 57, a former Libyan intelligence agent who has terminal cancer, was released on compassionate grounds by Scotland last week and returned to Libya to a jubilant welcome, despite American demands that he not be treated like a hero.

Mr. Obama called the release a “mistake,” and Robert Gibbs, the White House spokesman, called the arrival scene in Tripoli “outrageous and disgusting.” Mr. Megrahi had served 8 years of a 27-year-minimum sentence.

Administration officials said the next hurdle for the United States would be at the United Nations on Sept. 24, when the 15-nation Security Council is to meet on the subject of nuclear proliferation and disarmament. As part of a regular rotation, the United States will lead the session; Libya is one of 10 nonpermanent members on the Council.

Administration officials said that Mr. Obama, who is to serve as the chairman of the meeting, had no plans to meet with Colonel Qaddafi, who is expected to represent Libya at the session. But they predicted discussions within the administration about how to manage the choreography of the session. Part of the meeting is to be in an open chamber where every word and gesture will be covered by the news media.

“If he happens to be in the same forum, there’s very little we can do about it,” said another administration official.

nytimes