President Reagan urged visiting Indian Prime Minister Rajiv Gandhi to step up efforts with Pakistan to curb the spread of nuclear weapons in the region and received a pledge from Gandhi that India will not build such weapons "unless constrained to do so." Reagan also said U.S. aid to Pakistan is "not directed at India." The United States last year approved a new $4-billion package of military and economic aid for Pakistan. Payments were suspended after the lapse of a waiver for Pakistan under a U.S. law that bars aid to countries suspected of developing nuclear weapons. Reagan Administration officials are pushing for a new one-year waiver, which would require congressional approval. Pakistan has denied it is building nuclear weapons.