U.S. President Donald Trump on Thursday expressed ambivalence over a proposal to build a tunnel from Russia to Alaska, while Ukrainian President Volodymyr Zelenskyy voiced clear opposition to the idea.

"That's an interesting one," Trump told reporters during a meeting at the White House. "We'll have to think about that."

When asked for his opinion, Zelenskyy, who was in Washington to discuss ways to end the three-year conflict with Russia, said "I'm not happy with it."

The so-called "Putin-Trump Tunnel," proposed by Kirill Dmitriev, President Vladimir Putin's investment envoy and head of Russia's RDIF sovereign wealth fund, has recently drawn attention in some media reports.

Russia and the United States should build a "Putin-Trump" rail tunnel under the Bering Strait to link the two countries, unlock joint exploration of natural resources and "symbolize unity,"  the Kremlin envoy has suggested, envisaging a construction project costing $8 billion funded by Moscow and "international partners" to build a 112 kilometer rail and cargo link in under eight years.

Dmitriev, who has helped spearhead a Russian charm offensive designed to revive U.S.-Russia ties, floated the tunnel idea late on Thursday after Putin spoke to Trump by phone and agreed to meet in Budapest to try to find a way to stop the conflict in Ukraine.

The Bering Strait, which is 82 kilometers wide at its narrowest point, separates Russia's Chukotka region from Alaska. Ideas to link the two points have been around for at least 150 years, with various projects drawn up but never implemented.

Dmitriev, who has forged ties with Steve Witkoff, Trump's special envoy, and spoken of the possibility of U.S. energy majors taking minority stakes in Russian projects in the Arctic, suggested the tunnel could be built by The Boring Company, a U.S. tunnel construction company owned by Elon Musk.

There was no immediate public response to the idea from Musk.

(With input from agencies)