President Reagan asked the Senate on Friday to ratify a United Nations treaty against torture and ``demonstrate unequivocally our desire to bring an end to the abhorrent practice.'' Reagan said, however, that if the treaty is ratified he will declare that the United States does not recognize the competence of an international committee it sets up to investigate charges of systematic torture in the United States. ``I believe that a final United States decision as to whether to accept such competence of the committee should be withheld until we have had an opportunity to assess the committee's work,'' the president said. The treaty was unanimously adopted by the U.N. General Assembly in 1984 and was signed by the United States last month. ``It marks a significant step in the development during this century of international measures against torture and other inhuman treatment or punishment,'' Reagan said. ``Ratification of the convention by the United States will clearly express United States opposition to torture, an abhorrent practice unfortunately still prevalent in the world today.'' The treaty requires each signatory country to either prosecute torturers who are found in its territory or extradite them to other countries for prosecution. ``By giving its advice and consent to ratification of this convention, the Senate of the United States will demonstrate unequivocally our desire to bring an end to the abhorrent practice of torture,'' the president said. Reagan also submitted to the Senate for ratification a treaty signed in Montreal in February, under auspices of the International Civil Aviation Organization, providing for international cooperation in combatting terrorism at airports.