Geneva - The United Nations' top torture expert warned US President Donald Trump on Monday that endorsing waterboarding and other abuse as an interrogation technique would be illegal and have disastrous global consequences.Trump has repeatedly said he supports the use of torture in interrogations, although he has said he would defer to his Pentagon chief James Mattis, a torture opponent, for guidance on any new waterboarding push.His comments have nonetheless sparked alarm that he will try to reverse laws put in place by his predecessor Barack Obama outlawing the brutal interrogation techniques like waterboarding used by the CIA on suspects following the September 11, 2001 attacks."If the new administration were to revive the use of torture, the consequences around the world would be catastrophic," Nils Melzer, the UN special rapporteur on torture, said in a statement."Should Trump follow through on all of his pledges, more countries are likely to follow his lead and get back into the torture business," he warned, adding that this would be "an ultimate disgrace for all of humanity".Melzer said he was open to engage in "constructive dialogue" with Trump, but urged him not to reintroduce interrogation methods "that are more closely associated with barbarism than with civilisation".He pointed out that torture as an interrogation technique had repeatedly been shown to "produce false confessions and unreliable or misleading information"."Faced with the imminent threat of excruciating pain or anguish, victims simply will say anything - regardless of whether it is true - to make the pain stop and try to stay alive," he explained.Moreover, torture is neither legally nor morally acceptable, Melzer said."If you are looking for military advantages in war, you can argue that chemical weapons 'work', or terrorism 'works' as well," he warned.And under international law, the prohibition against torture is "absolute", he said, stressing that when used in the context of armed conflict such techniques could amount to war crimes.