The Philippines faced calls Friday to investigate its firebrand president after a self-confessed hitman alleged Rodrigo Duterte ordered a thousand opponents and suspected criminals murdered when he was a city mayor. Edgar Matobato told a Senate inquiry on Thursday that he and a group of policemen killed some 1,000 people in Davao city on Duterte's orders from 1988-2013, with the politician himself shooting dead one of the victims. "These are serious allegations and we take them seriously, we look into them," said US State Department deputy spokesperson Mark Toner. The allegations surfaced as the Senate investigated alleged extra-judicial killings in an ongoing anti-drug crackdown that has led to more than 3,000 deaths in Duterte's first 72 days in office. Critics say the alleged killings in Davao, where Duterte was mayor for more than 20 years, established a pattern that has spread nationwide under the new presidency. The testimony of self-confessed hitman Edgar Matobato sheds light on "the similarity of the strategy adopted by the (Davao Death Squad) and that of the vigilantes that now roam the whole country," Senator Leila de Lima, leading the inquiry, said in a statement. US-based watchdog Human Rights Watch urged Manila to let United Nations investigators probe the hitman's claims. "President Duterte can't be expected to investigate himself, so it is crucial that the United Nations is called in to lead such an effort," the monitor's Asia director Brad Adams said. Sitting Philippine presidents are immune from criminal prosecution during their single, six-year term. However, the constitution provides for their impeachment and removal from office for "culpable violation of the constitution, treason, bribery, graft and corruption, other high crimes, or betrayal of public trust". In 2001, president and populist ex-movie star Joseph Estrada was removed from office in a military-backed popular revolt, though an impeachment trial against him on graft charges was inconclusive. During his election campaign at the start of the year, Duterte variously admitted and denied involvement in the death squads. He has so far ignored the latest allegations but Justice Secretary Vitaliano Aguirre branded them as "lies and fabrications". - 'Killings haven't stopped' - Another Duterte ally, Senator Alan Cayetano, alleged Thursday that the inquiry was part of an opposition "Plan B" to unseat the president -- a charge de Lima rejected. However, she later suggested it may be time to "revisit" the presidential immunity doctrine. "Otherwise there will be no solution but impeachment, people power, things like that," she told reporters Thursday, asking: "What if we had elected a mass murderer, serial killer or rapist?" Wilnor Papa, a campaign officer for the Manila office of Amnesty International, said rampant killings were the outcome of the failure of previous governments to bring criminal charges against Duterte. "We are now seeing riding-in-tandem (motorcycle-borne assassins) like those that prowled the Davao streets in the late 1990s. The targets are not only drug syndicates. Even purse snatchers use them and they can target basically anyone," he told AFP. House of Representatives member Edcel Lagman urged Duterte Friday to name an independent fact-finding commission made up of retired judges to "determine the identities of the principals and perpetrators as well as of the victims". Catholic priest Amado Picardal, a critic from Davao, said the city assassins killed 1,424 people between 1998 and 2015, mostly in slums with the victims including 132 children and two journalists. Most victims were either involved in illegal drugs or petty crimes. All were unarmed, did not fight back and were "shot in cold blood", he wrote in the Catholic Bishops' Conference of the Philippines online newsletter. "The killings have not stopped," he said. The fate of the former death squad member Matobato was meanwhile uncertain on Friday as the Senate president, Duterte ally Aquilino Pimentel, refused to take him into protective custody. There's "no (sign) that his life or safety is threatened," Pimentel told AFP.