The United States is expected to impose additional penalties on Ukraine in response to its alleged sale of a sophisticated radar system to Iraq in violation of U.N. economic sanctions, a U.S. official said Thursday.

The U.S. case against Ukraine is based almost entirely on a July 2000 tape recording in which President Leonid Kuchma reportedly approved the Kolchuha radar deal in a telephone conversation.

After official tests confirmed that it was Kuchma's voice on the recording, the Bush administration reduced U.S. aid to Ukraine by $54 million.

The Ukrainian government has denied transferring the system to Iraq, insisting its arms exports are regulated by controls created with U.S. and other Western assistance.

The United States and Britain recently sent a team to Ukraine in search of additional information but the visit proved to be inconclusive, the official said.

But, the official said, the administration has deemed that the taped phone conversation was proof enough.

Before the aid cut, the former Soviet republic was the fourth-largest recipient of U.S. assistance, receiving some $230 million annually.

The Kolchuha system can detect U.S. and British aircraft that patrol "no-fly" zones in northern and southern Iraq. The detection occurs without the pilots knowing they are being monitored.

Amid the U.S. allegations against Kuchma, NATO declined to invite him to its summit in Prague in November. NATO plans to go ahead with ministerial-level meetings with Ukraine during the gathering in the Czech capital. Ukraine is not as member of NATO but consults periodically with NATO officials on security issues.

Ukraine's foreign minister, Anatoliy Zlenko, threatened Thursday to boycott the summit. He accused Washington of campaigning to deny Kuchma a place at the meeting and said its "misguided" policy jeopardized Ukraine's bid to get on track to join the alliance.

Patricia Guy, spokeswoman at the U.S. Embassy in Kiev, said the United States supports NATO's decision not to include Kuchma but remains "strongly committed to Ukraine's integration with NATO."