Paul Ryan, the US House speaker, said on Wednesday that Donald Trump should not pardon himself and pointed out that “no one is above the law” – becoming the most senior Republican in Congress to speak out against the president’s assertion that he has the “absolute” power to do so.

Ryan also backed another senior Republican’s assertion that there is no evidence that the FBI planted a “spy” in Trump’s 2016 presidential election campaign when it sent a secret informant to investigate signs of potentially improper links with high-level Russian operatives.

Ryan was asked on Wednesday by reporters at the US Capitol whether Trump can legally pardon himself, most pertinently if he ends up falling foul of the Russia investigation by special counsel Robert Mueller that evolved out of that early scrutiny.

Ryan said: “I don’t know the technical answer to that question but obviously he shouldn’t. No one is above the law.”

Trump made the pardon assertion in a series of Twitter posts on Monday in which he again criticized Mueller’s criminal investigation, which has been going on for just over a year, of whether his 2016 presidential campaign colluded with Russia to sway the election in his favor and whether there has been any subsequent obstruction of justice.

“As has been stated by numerous legal scholars, I have the absolute right to PARDON myself, but why would I do that when I have done nothing wrong?” Trump posted.

In another tweet he called Mueller’s appointment by the Department of Justice in 2017 “totally UNCONSTITUTIONAL”.

Trump has denied he colluded with the Russians, obstructed justice or committed any wrongdoing, and repeatedly calls Mueller’s investigation a “witch-hunt.”

Ryan, who is not seeking re-election, has avoided taking a confrontational approach with the president after having had an often-strained relationship with him. But on Wednesday he became the second senior congressional Republican to contradict the president’s controversial declaration.

The US Senate’s Republican leader, Mitch McConnell, also discouraged talk of a self-pardon by Trump on Tuesday.

“Let me just tell you that from – from my perspective – I don’t think the president needs any advice on pardoning himself,” McConnell told reporters. “He obviously knows that would not be something that he would or should do.”

On Wednesday, Ryan agreed with a key House Republican who says there’s no evidence that the FBI planted a “spy” in Trump’s 2016 campaign.

The comments contradict Trump, who has insisted the agency planted a “spy for political reasons and to help Crooked Hillary win”.

Ryan and Trey Gowdy, the House oversight and government reform committee chairman, attended a classified briefing last month following reports that the FBI used an informant in its Russian meddling investigation. Gowdy said afterwards he was convinced that “the FBI did exactly what my fellow citizens would want them to do” and he did not see evidence of a spy.

Ryan said Gowdy’s “initial assessment is accurate” and he had seen no evidence to the contrary. But he said Congress still has questions about the issue.

Also on Wednesday morning, speaking while on a trip to Israel, Rudy Giuliani, who is one of Trump’s legal team, said Mueller was trying to “frame” Trump with his investigation, as a result of partisan motivations. He has previously admitted that Trump’s attempted branding of the FBI’s initial scrutiny of his campaign as “Spygate” is a public relations tactic.