Amid signs of stepped-up diplomatic contact, the White House disclosed Monday that officials of the United States and the Palestine Liberation Organization met during the weekend to discuss issues that included the bombing of Pan American Flight 103.

White House spokesman Roman Popadiuk said Robert Pelletreau, the U.S. ambassador to Tunisia, who opened a formal dialogue with the PLO Dec. 16, met Saturday at the U.S. Embassy in Tunis with Hakam Balaawi, the PLO representative to Tunisia and the Arab League.The disclosure coincided with a flurry of news reports about offers of PLO assistance in tracking down those responsible for the Dec. 21 bomb blast that brought down Pan American Flight 103 over Scotland and of unexpected plans for a resumption of formal talks between the United States and the PLO before the Jan. 20 inauguration of President-elect George Bush.

However, Popadiuk characterized the weekend meeting in Tunis as "informal" and said it was requested by Balaawi "for the purpose of introducing the latter to the ambassador."

"Discussion centered on Middle Eastern issues," Popadiuk said. "Since the meeting was called by Mr. Balaawi, the ambassador was principally in a listening mode."

At the same time, he noted that Pelletreau "took the opportunity to mention that the U.S. would welcome any information the PLO is able to develop concerning the Pan Am tragedy."

"This is in line with what we have already publicly stated," Popadiuk said, "and is true for the PLO as well as anyone else who may have information on the bombing."

An administration official emphasized that Pelletreau made no direct request for PLO assistance in tracking down those responsible for the bombing, which President Reagan and others have classified as a suspected act of terrorism.

The official said Balaawi, who was not among the PLO officials who met with Pelletreau Dec. 16, "took note of the ambassador's view and said he would pass it on" to PLO chairman Yasser Arafat.

President Reagan has said he would be happy to receive any information that Arafat might offer to help solve the mystery.

Meanwhile, reports that a Lebanese-born American student may have unwittingly carried the bomb that destroyed Pan American Flight 103 have been discredited following the discovery of part of the youth's luggage, London's Independent newspaper said Tuesday.

A civilian involved in search operations in the Scottish countryside told the newspaper remnants of Khalid Jaafar's baggage were found in the hills near Langholm, about eight miles east of Lockerbie, Scotland, on Saturday.

Part of the zipped dufflebag contained the 21-year-old's identity card and a roll of money from a country in the Middle East. But the luggage did not bear burn marks or other damage that might have indicated it was close to the site of the explosion, the worker said.

On Tuesday, the Soviet Union said it will help track down the terrorists who planted the bomb.