VIENNA, Austria (AP) _ East-West talks on reducing troop strength in central Europe resumed Thursday after an eight-week Easter recess. The negotiations have stretched over nearly 12 years, with no signs of progress.

NATO spokesman John Karch of the United States told reporters that President Reagan had expressed continuing interest and support for the Mutual and Balanced Force Reduction talks.

He said Reagan told the new chief U.S. delegate, Robert D. Blackwill, at a May 21 meeting in Washington to ''probe for all possible areas of agreement.'' Blackwill replaced Maynard Glitman, now in Geneva, Switzerland, to negotiate with the Soviets on intermediate-range nuclear weapons.

Asked whether Reagan's instructions indicated heightened interest in the talks, Karch said they were ''not unusual. ... The remarks speak for themselves.''

He said NATO was considering recent suggestions by the Soviet bloc's Warsaw Pact and promised an answer ''in due course.'' He complained of ''negative Eastern response'' to Western arguments.

Warsaw Pact spokesman Ulrich Potrafky of East Germany challenged the West to provide what he called a constructive answer to the proposals.

The two sides have disagreed for years on how many troops each really has in central Europe and how to verify withdrawals. NATO says its soldiers are outnumbered by about 200,000 and the Warsaw Pact says there is rough parity at about 990,000 ground and air personnel each.

Initial cuts of 20,000 Soviet troops and 13,000 U.S. soldiers were suggested in the latest Warsaw Pact proposal. The West rejected a similar idea in 1983, insisting on agreement about troops numbers before reductions are considered.

''There should be mutual understanding of the forces involved,'' Karch said in a prepared statement. ''Only this can provide an agreed basis for calculating necessary reductions, monitoring interim and final ceilings and resolving any possible disputes over compliance.''

The negotiations, which are held in private, began Oct. 30, 1973.

North Atlantic Treaty Organization members are the United States, Canada, Britain, West Germany, Belgium, Italy, the Netherlands, Luxembourg, Norway, Denmark, Greece and Turkey.

The Soviet Union, East Germany, Poland, Czechoslovakia, Hungary, Romania and Bulgaria are in the Warsaw Pact.