The United States on Monday signaled that it was softening on what has become an effective oil blockade on Cuba, an about-face after President Trump spent weeks threatening to take over the island and saying he would punish nations that shipped fuel there.

Speaking to reporters, Karoline Leavitt, the White House press secretary, said the United States would evaluate oil shipments to Cuba on a “case-by-case basis.” She was responding to questions about why the U.S. Coast Guard allowed a Russian tanker full of crude to reach Cuba, even though it has been blocking other countries from doing the same.

“It will continue to be made on a case-by-case basis for humanitarian reasons or otherwise, but there’s been no firm change in our sanctions policy,” Ms. Leavitt said.

The Trump administration has blocked energy shipments to Cuba since January as part of a strategy to force the Communist government into submission. Mr. Trump said in a January social media post: “THERE WILL BE NO MORE OIL OR MONEY GOING TO CUBA — ZERO!”

But when it came to an oil tanker from Russia, Mr. Trump — who has long expressed admiration for President Vladimir V. Putin of Russia — made an exception. For Russia, Mr. Trump’s decision to let the ship through came as the latest demonstration that it has some leverage over Washington.

Dmitri S. Peskov, Mr. Putin’s spokesman, said the fuel shipment had been discussed in advance with the United States and that Russia had ⁠a duty to support “friends” in Cuba.