Soviet criticism of President Carter has increased sharply in the past few days, and for the first time has extended to his positions on such vital issues as startegic arms limitation and Middle East settlement.

Criticism of an American president by name in the press here has been unusual in recent years, but Carter's outspoken stand in support of human-rights activists in Eastern Europe had already prompted angry words from Kremlin spokesmen.

The decision now to criticize the President's statements on SALT and the Middle East probably reflects concern in the Soviet leadership that the problems with the new administration may be more serious than the squabble over dissidents.

The Kremlin's concern at the human-rights confrontation had appeared to be tempered somewhat by its expectation that quick headway could be made on what it considers the core issues of detente, especially since it sees the human-right issue as basically an ideological matter with no direct bearing on international tensions.

SALT and the Middle East, on the other hand, rate as the two most important subjects on the superpower agenda as the Soviets see it.

The key question is whether the Kremlin will let its public dismay over Carter's initial moves on these crucial matters influence private negotiations with the administration.

Secretary of State Cyrus R. Vance's Moscow visit later this month - the first high-level contact between the superpower since Carter took office - was always regarded as an event of importance; but as the atmosphere for those talks worsens, the challenge Vance faces inassuring Moscow of Carter's interest in detente grows greater.

On SALT, the Soviets have clearly been disapproving of Carter's proposal to eliminate the troublesome U.S. cruise missile and Soviet Backfire bomber from an agreement to break the deadlock on the role of those weapons that has existed for nearly a year.

"One cannot fail to note" Pravda, the Communist Party newspaper observed today, "that this is a distinct departure from the previous U.S. stand" - a reference to the Vladivostok understandings of November 1974, which the Soviets maintain are the only basis for a strategic arms pact.

Reaction to Carter's remarks last week during and after the Washington visit of Israeli Prime Minister Yitzak Rabin have been similarly negative. In a commentary entitled "Unjustified Hopes," the official Tass news agency said Carter had aligned himself with "Israel's refusal to return all terroritories occupied during its aggressions" - criticism of the president's contention that Israel should have "defensible borders" as the outcome of a regional settlement.

Another Tass dispatch - although less authoriative - went even further, quoting Arab newspapers as saying that Carter's statement "reaffirmed in the best possible way the imperialist aggressive essence of American policy in the Middle East."

The Soviets and their allies have also kept up their criticism of Carter's position on human rights: "Is the White House not aware," the Polish newspaper Zycie Warszawy wrote, and was prominently quoted in Pravda today, that its interference in the internal affairs of other countries . . . greatly complicates talks aimed at easing international tensions, "which are already difficult enough."