Greece will tell the United States by the end of the year to start closing U.S. military bases, but negotiations for a new agreement will continue into 1989, the government announced Friday.

Government spokesman Sotiris Kostopoulos said the current five-year operating agreement for the four American bases ends on Dec. 20, and "the 17-month removal period begins.""During that period the conditions of the agreement remain in force," Kostopoulos said in a statement.

The statement was issued after Greek and American negotiators ended a 12th round of talks in Athens on Friday.

Negotiations for a new bases agreement began in November 1987 but have made little progress.

The spokesman said negotiations will continue into 1989 and "during the entire 17-month removal period to ascertain the margins existing for a possible new agreement." He said another round of talks will begin in January.

Kostopoulos said that in this week's talks, although "a deadlock was not reached, a considerable gap in views continues to exist on many issues."

In Washington, a State Department official said, "Important differences remain, but the negotiations are not at an impasse." The official spoke on condition of anonymity.

Talks broke off in September when Greece said that Hellenikon Air Force base adjacent to Athens airport would not be included in a new agreement and would have to close.

Negotiations resumed in October after Foreign Minister Carolos Papoulias discussed the issue with Secretary of State George P. Shultz in Washington.

Kostopoulos' statement noted Greece's decision to shut down Hellenikon.

About 1,400 servicemen, 200 civilian employees and 700 Greeks work at the base, set up in the late 1940s to help distribute Marshal Plan aid to war-torn Greece.

Hellenikon has been an irritant to nationalist and leftist groups opposed to the U.S. presence in Greece.

The spokesman also repeated a Greek demand that a new agreement would have to serve Greece's "highest national interests" and said a new accord would have to be approved in a national referendum before it is signed.