US Senate Minority Leader Chuck Schumer said on Sunday that senior administration officials must testify in Donald Trump’s impeachment trial, after a report alleged the president told a former adviser to suspend military aid to Ukraine pending investigations into political rivals. In a tweet, Mr Schumer urged Republicans to support efforts to call former national security adviser John Bolton, among others, to testify. Mr Schumer was referring to Mr Bolton's unpublished manuscript. <em>The New York Times</em> reported on Sunday that it reveals Mr Trump told Mr Bolton in August he wanted to continue withholding $391 million (Dh1.43 billion) in aid until Ukrainian officials provided information about investigations into former US vice president Joe Biden, among others. The White House did not immediately comment on the manuscript, nor did lawyer Jay Sekulow, who is helping lead Mr Trump’s defence in his impeachment trial in the US Senate. Charles Cooper, a lawyer representing Mr Bolton, did not respond to a request for comment. The revelations could undercut one of the primary defences made by Mr Trump and his allies amid the impeachment inquiry – that the hold on aid was unrelated to any desire to see Ukraine launch investigations into political opponents, including Mr Biden, whose son Hunter was director of a Ukrainian energy company during his father’s time as vice president. According to the <em>Times</em>, Mr Trump was pressed for weeks by senior aides, including Mr Bolton, Secretary of State Mike Pompeo and Defence Secretary Mark Esper to release the aid, which had been appropriated by Congress. The White House has directed Mr Bolton and other administration officials not to co-operate with the impeachment probe, although Mr Bolton has said he would testify on the matter if subpoenaed. The Senate is hearing arguments for removing Mr Trump from office on charges of abuse of power and obstruction of Congress stemming from the Republican president’s dealings with Ukraine. But it is not clear whether there will be sufficient support among senators to call for additional witnesses and evidence. The House did not receive testimony from Mr Bolton.