Soviet President Konstantin Chernenko was reported by sources here to be ill, and the Tass news agency announced early today that a Warsaw Pact summit meeting Chernenko had planned to attend this week in the Bulgarian capital of Sofia would be postponed.

Details about Chernenko's health problems were not revealed by the sources, but it is known that the 73-year-old Soviet leader suffers from respiratory problems.

Extremely low temperatures prevented his appearance last month at the Red Square funeral of defense minister Dmitri Ustinov. Chernenko had kept a busy schedule through December, however.

There have been persistent rumors in the diplomatic community during the past few days that the Sofia summit would be postponed.

There have been suggestions here that the postponement may be due to the changes at the top of the Soviet military hierarchy following Ustinov's death, but most diplomats believe that it was linked to Chernenko's health.

[Western journalists in Sofia also reported widespread rumors about the Soviet leader's health and said there had been few signs that the Bulgarian capital was preparing to host a major gathering of Soviet Bloc leaders, Reuter reported.]

According to an official statement, Chernenko attended the last meeting of the Politburo on Thursday, when the ruling council discussed and approved a report by Foreign Minister Andrei Gromyko about his talks with Secretary of State George P. Shultz in Geneva last Monday and Tuesday.

The statement said that the Politburo also discussed and approved Chernenko's proposals dealing with unspecified "basic questions" to be prepared for the ruling council.

An official statement distributed by the Soviet news agency Tass early today said that the summit meeting due to open in mid-January has been postponed "till a later date, which is to be agreed upon." The announcement gave no reason for the postponement.

The summit was to have included the Communist Party leaders, premiers, defense and foreign ministers of the Soviet Union, Poland, Czechoslovakia, East Germany, Hungary, Romania and Bulgaria.

The visit to Sofia was to have been Chernenko's first trip abroad since he became general secretary of the Soviet Communist Party last February.

Bulgarian diplomats had said earlier that the Soviet leader was to have stayed on for two additional days in Bulgaria on a state visit following the summit.

The last Warsaw Pact summit, known as the meeting of the Political Consultative Committee of the military alliance, was held in January 1983 in Prague under the chairmanship of Chernenko's predecessor, the late Yuri Andropov.

The Soviet Bloc leaders gathered again in Moscow in June 1983, but that meeting was not described as a pact summit.

The announcement that the summit would be held in Sofia in mid-January was made by the official news agency Tass on Dec. 28. It was also announced by the armed forces newspaper Krasnaya Zvezda (Red Star) on Dec. 29.