Russian President Vladimir Putin joked with President Trump Friday that special counsel Robert Mueller’s investigation of alleged collusion between Moscow and the Trump campaign “started off as a mountain and ended up as a mouse.”

“He actually sort of smiled when he said something to the effect that ‘it started off as a mountain, and it ended up being a mouse,’ but he knew that, because he knew there was no collusion whatsoever,” Mr. Trump told reporters, describing his lengthy talk with the Russian leader.

An aide said later that it was not a video call, but that Mr. Trump was describing how the Russian leader “chuckled” over the phone.

The president said they discussed topics ranging from Venezuela to nuclear weapons treaties to the “Russian hoax” of alleged campaign collusion.

He said Mr. Putin claims that he “doesn’t want to get involved” in Venezuela, other than “he’d like to see something positive happen” for the country. The U.S. blames Russia for propping up socialist leader Nicolas Maduro amid a worsening economic crisis and street clashes between government forces and people supporting U.S.-backed interim President Juan Guaido.

“We want to help on a humanitarian basis, and I thought it was a very positive conversation I had with President Putin on Venezuela,” Mr. Trump said.

The president also said he discussed with Mr. Putin “the possibility of a three-way deal” with China on reducing nuclear arms, and revealed that he has already spoken with Chinese leaders about it. He said they spoke about “extending the New START [treaty] or adding China to it.”

The U.S. and Russia are parties to the 2010 New Strategic Arms Reduction Treaty that limits deployed strategic weapons, but does not include China. The pact is due to expire in 2021 but could be extended for up to five years.

“We’re talking about a nuclear agreement, where we make less [weapons] than they make less, and maybe even when we get rid of some of the tremendous firepower that we have right now,” Mr. Trump said. “China … very much would like to be a part of that deal. They were excited about that, maybe even more excited [than] about trade. So I think we’re going to probably start up something very shortly between Russia and ourselves … and I think China will be added down the road. We’ll be talking about nonproliferation, we’ll be talking about a nuclear deal of some kind, and I think it will be a very comprehensive one.”

A week after Mr. Putin met with North Korean leader Kim Jong-Un, the president said he and the Russian “talked about North Korea at great length.” He didn’t elaborate.

Mr. Putin met with Mr. Kim in Vladivostok, Russia, signaling that he wants to be a player in any talks aimed at getting Pyongyang to abandon its nuclear weapons program.

Mr. Trump held his second denuclearization summit with Mr. Kim in late February, a meeting that ended without any agreement.

Mr. Trump said he and Mr. Putin also discussed increasing trade between the U.S. and Russia.

“We intend to do a lot of trade with Russia,” he said. “We do some right now, it’s up a little bit, but he’d like to do trade, and we’d like to do trade.”