The head of the CIA offered stern warnings for President-elect Donald Trump on Wednesday about sabotaging the nuclear deal with Iran, warming up to Russia and returning to brutal forms of interrogation widely considered to be torture.

Those steps, which Trump has promised to pursue as president, would embolden foreign hard-liners and meet heavy resistance from within the CIA, Director John Brennan said in an interview with the BBC.

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Tearing up the Iran deal, Brennan said, “would be disastrous.”

“It would be the height of folly if the next administration were to tear up that agreement," he claimed, warning that other governments would use the development to start pursuing nuclear weapons of their own.

Trump has repeatedly criticized the accord, which sets limits on Iran’s nuclear program in exchange for the lifting of sanctions.

He has called it “the worst deal ever negotiated” and said that, as president, he would force Iran to return to the table or else risk the agreement falling apart.

Brennan’s position is at sharp odds with that of Rep. Mike Pompeo (R-Kan.), the lawmaker who Trump intends to nominate to replace Brennan atop the CIA. Pompeo has been one of Congress’s most vocal critics of the nuclear agreement, and he previously traveled to the United Nations’ nuclear regulatory headquarters in Vienna to try and shed light on secret components related to the pact.

The president-elect has also promised to renew the CIA’s former controversial use of so-called enhanced interrogation techniques, such as waterboarding and sleep deprivation. The methods are widely considered torture and are now illegal. Trump would likely not be able to legally reinstate the practices without an act of Congress.

He also might meet resistance from within the CIA, said Brennan, part of whose tenure at the spy agency has been defined by the shadow of the President George W. Bush-era practices.

"Without a doubt the CIA really took some body blows as a result of its experiences," Brennan told the BBC. "I think the overwhelming majority of CIA officers would not want to get back into that business."

Pompeo has not called for resumption of the brutal techniques, though he harshly criticized Senate lawmakers after the 2014 release of a scathing report about the program. The full text of that 6,700-page report remains classified, although Sen. Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.) — who devoted years to finalizing the report — has pushed the White House to make it public before President Obama leaves office.

In his interview with the BBC, Brennan also cautioned Trump against developing a close relationship with Russian President Vladimir Putin, about whom the president-elect has been remarkably complimentary.

"I think President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE and the new administration need to be wary of Russian promises,” Brennan said.

Moscow has posed a consistent obstacle to the Obama administration’s plans to stem the civil war in Syria and has been “disingenuous” during efforts to work together, the CIA head claimed.

Hackers with ties to the Russian government are also alleged to have penetrated political organizations in the U.S., including the Democratic National Committee, in the run up to Election Day this year.

The Obama administration’s extraordinary disclosure of Russia’s involvement in the hacks has raised questions about how the U.S. should respond and whether it would be advisable to retaliate in kind.

Americans should not “stoop to their level,” Brennan told the BBC, in part because of fears about an escalating cyberwar. Instead, he urged other means of retribution, which he did not outline.