President Trump ripped a treaty that capped American and Russian deployment of nuclear warheads as a bad deal for the US during his phone call last month with Russian leader Vladimir Putin, it was reported Thursday.

And when Putin suggested extending the 2010 treaty, known as New START, Trump had to pause to ask his aides what the treaty was about, Reuters reported, citing two current and one former US official.

Trump’s move came despite Secretary of State Rex Tillerson telling senators during his confirmation hearing that he supported extending the treaty.

Trump told Putin the 2010 treaty was one of a number of bad deals negotiated by the Obama administration — and also bragged about his own popularity during the call, Reuters said.

Team Trump did not immediately respond to a request for comment.

They referred Reuters to the official White House account issued after the Jan. 28 call, which did not mention the discussion about New START.

New START gives both countries until February 2018 to reduce their deployed strategic nuclear warheads to no more than 1,550, the lowest level in decades.

The treaty also limits deployed land- and submarine-based missiles and nuclear-capable bombers.

During a debate in the 2016 presidential election, Trump said Russia had “outsmarted” the United States with the treaty, which he then called “START-Up.”

He said incorrectly then that it had allowed Russia to continue to produce nuclear warheads while the US could not.

During his confirmation hearings, Tillerson said it was important for the US to “stay engaged with Russia, hold them accountable to commitments made under the New START and also ensure our accountability as well.”

Typically, before a telephone call with a foreign leader, a president gets an in-depth briefing paper drafted by National Security Council staff after consultations with the relevant agencies, including the State Department, Pentagon and intelligence agencies, two former senior officials said.

Just before the call, the president also usually receives an oral “pre-briefing” from his national security adviser and top subject-matter aide, they said.

But Trump was not briefed by Russia experts with the NSC and intelligence agencies before the Putin call, two of the sources told Reuters.

Reuters was unable to determine if Trump received a briefing from his national security adviser, Michael Flynn.

In the phone call, the Russian leader raised the possibility of reviving talks on a range of disputes and suggested extending New START, the sources said.

New START can be extended for another five years, beyond 2021, by mutual agreement. Unless the two sides agree to do that or negotiate new cuts, the world’s two biggest nuclear powers would be freed from the treaty’s limits, potentially setting the stage for a new arms race.

New START was ratified by the Senate in December 2010 by a vote of 71-26. Thirteen Republican senators joined all of the upper house’s Democrats in voting for the treaty, although Republican opponents called it naive.