Some 12,000 scientists, engineers and other professionals warned in a declaration yesterday that the continuing East-West nuclear arms race "is increasingly a mortal threat to all humanity."

The Union of Concerned Scientists called on the United States to take the intiative in halting the testing and deployment of new nuclear weapons and delivery system. They said Russia should reciprocate.

The union is a non-profit organization based at Cambridge, Mass. Supporters of the statement include 21 Nobel Prize laureates.

The three-page declaration said that if the inventory of East-West nuclear warheads and delivery systems were used in war it "would kill hundreds of millions of persons, carry radioactive injury and death to many of the world's nations, profoundly damage the environment of the earth we live and depend on, and unhinge and devastate the target nations so effectively that they would no longer function as modern industrial states."

The declaration said the United States should.

Announce it will halt underground testing of nuclear explosives provided that the Soviet Union follows suit within a reasonable time.

"Announce that it will not field test or deploy new strategic nuclear weapons, nuclear weapons systems, or missile defense system for a period of two to three years provided the Soviet Union demonstrably does likewise."

It said "these actions, if carried out by the United States, would represent a policy of restraint of the greatest political significance and yet, for an interim period, be devoid of military risk."