WASHINGTON (AP) _ The Reagan administration, dismissing the doubts expressed by several congressmen, said Thursday it remained certain the Soviet Union was violating the Anti-Ballistic Missile treaty by constructing a long-range radar in east central Russia.

Two ranking Pentagon officials, appearing at a special briefing, added they thought the Soviets had scored a propaganda victory by allowing three congressmen to visit the radar construction site at Krasnoyarsk last week.

''My personal view is that it was a very skillful propaganda stroke for the Soviet Union and it has not advanced the interests of the United States,'' said Frank J. Gaffney, the acting assistant defense secretary for international security policy.

''From a Soviet standpoint, this was nothing but a win, because here they can appear forthcoming without telling us anything,'' added James McCrery, a ranking civilian official with the Defense Intelligence Agency.

''They can bring people into a radar that is just in the late midphase of construction. The detailed components ... aren't in there yet. Not much more is visible from the inside than the Americans probably know already from the outside.''

Earlier this week, the three congressmen who visited Russia told reporters their visit to the radar site had left them uncertain as to whether it violated the 1972 ABM treaty.

''This was an extraordinary step in confidence-building, an extraordinary visit for arms control,'' added Rep. Thomas Downey, D-N.Y., in a statement echoed by Reps. Jim Moody, D-Wis., and Bob Carr, D-Mich.

The three spent a day last week inspecting the Soviet radar system at Krasnoyarsk, a facility in the central Soviet Union which has been the focus of a superpower dispute. The visit was the first by foreigners ever allowed by the Russians.

The Reagan administration contends the radar is designed not only to detect incoming missiles, but also to track them closely - a purpose known as battle management. That type of radar system is banned by the 1972 treaty because the pact limits the deployment of systems that can be used to shoot down ballistic missiles.

The Soviets, on the other hand, argue the radar is for tracking objects in deep space, which would not be a violation of the ABM treaty.

In a report to House Speaker Jim Wright, D-Texas, the three congressmen concluded, ''Based on what we saw, we judge the probability of Krasnoyarsk functioning as a battle-management radar to be extremely low.''

Gaffney and McCrery adamantly disputed that finding, saying the radar clearly was being built to detect as well as track ballistic missiles. The facility is too large, too strategically placed and too similar to other known types of Soviet radars to have any other purpose, the two said.

''As such, it is a clear-cut violation of the ABM treaty,'' said Gaffney. ''And militarily, the Krasnoyarsk radar violation goes to the heart of the ABM treaty.''

Gaffney asserted Thursday that the evidence also remains overwhelming that Krasnoyarsk is merely one piece in an ever-growing network of advanced radars that could be used to support a full-blown, nationwide missile defense system inside the Soviet Union.

''Based on what we have seen so far (from the congressional trip), none of our judgments have changed,'' McCrery added.