A key adviser to Romanian President Nicolae Ceausescu, a central go-between in the early stages of Egyptian-Israeli peacemaking, held a secrecy-shrouded meeting today with Prime Minister Menachem Begin.

Israeli officials at first would not confirm that the meeting took place and then refused to disclose details of the talks. But it was learned that the Ceausescu emissary delivered a written message from the Romanian president during the 1 1/2-hour session.

Ceausescu, who helped arrange the first meeting here between Begin and Egyptian President Anwar Sadat, last month visited Syrian President Hafez Assad and Palestine Liberation Organization chief Yasser Arafat, after which he said he hoped to play a "positive and active role" in establishing a comprehensive Middle East peace.

An aide to Begin, Dan Pattir, side stepped questions about whether the visit is linked to a new Romanian peace initiative, or whether the emissary, Vasile Pungan, brought details of Ceausescu's talks with Assad and Arafat.

Pattir defined the talks as "discussions concerning Israeli-Romanian relations as well as issues of international relations concerning the Middle East that carry mutual interests to all parties."

A spokesman for the Romanian delegation in Tel Aviv also refused to comment on the talks, saying only that "Bucharest is very concerned about the delays in progress toward a solution of the Palestinian problem."

Romania is the only Soviet Bloc nation that maintains diplomatic ties with Israel. Diplomatic sources said that Ceausescu initiated the Pungan-Begin meeting.

Several hours after the meeting, Israel's Foreign Ministry denied any knowledge of it and referred inquiries to the prime minister's office. Foreign Minister Moshe Dayan was reported to have said, "I haven't the slightest knowledge of the arrival of any emissary from the president of Romania."

Pungan, it was learned, arrived here last night. Sources said there were no plans for further meetings during his stay.

Asked about the secrecy, Pattir said, "It was supposed to be a meeting that the Romanian side requested not have any publicity." He said he did not know the source of the leak.

Former prime minister Yitzhak Rabin said that during his recent meeting with Ceausescu in Bucharest he had no indication that a Romanian envoy planned to meet with Begin.

Pungan last year conducted several quiet diplomatic missions to Syria, Jordan and the PLO to promote Ceausescu's quest for Middle East peace.

Ceausescu, who in 1977 relayed preliminary messages between Sadat and Begin prior to Sadat's historic visit to Jerusalem, has consistently supported the creation of a Palestinian state and PLO recognition of Israel's right to exist.