As Ukraine Raises Alarm Over U.S. Backing, Europeans Pledge Support President Volodymyr Zelensky, who met with Britain’s prime minister, has said Ukraine received no warning of Russian attacks, but President Trump has said Washington is still “sharing intel.”European leaders vowed to redouble their support for Ukraine on Wednesday, amid fears that President Trump could stop or cut back on sharing military intelligence with Ukraine in its war with Russia.

President Volodymyr Zelensky of Ukraine on Wednesday referred to what he called recent “difficulties” with partners during a meeting in London with Prime Minister Andy Burnham of Britain. Mr. Zelensky did not offer details, but said he would talk with Mr. Burnham about intelligence sharing and Starlink, the satellite company owned by Elon Musk that has been crucial to its military.

He said there had been “a lot of different surprises” that wouldn’t be expected “when you have real partnership, and we count on partnership with the United States.”

On Wednesday, there was a sudden restriction of Ukraine’s access to a military version of the Starlink system, which is paid for by the Pentagon, security officials said. The loss of that access has not been explained.

The prospect of a disruption in the security relationship with the United States — which has enabled Ukraine to attack with precision and defend against incoming missiles and drones — rattled European officials, nearly five years into the war. Verbal clashes between Mr. Trump and Mr. Zelensky in recent days have further frayed nerves, raising concern about how Ukraine could continue to defend itself against Russia if it were to lose U.S. intelligence.

Mr. Zelensky said on Tuesday that his country had received no early warning from its security partners of Russian missile attacks on Monday night and again on Tuesday night. That raised concerns that the United States might have followed through on an apparent threat to halt intelligence like satellite data that can warn of missile launches.

But security officials in several countries, including the United States and Ukraine, said critical American intelligence was continuing. When asked about it on Tuesday, Mr. Trump said, “We are helping them, and we are sharing intel.”

A spokesman for the Ukrainian Air Force, Yurii Ihnat, said on Wednesday that Ukraine had received no warning for one set of Tuesday night strikes, but did get warning for the “next three waves of attacks.”Russia struck Ukraine’s energy infrastructure heavily overnight, causing significant damage to 26 power-grid facilities and four oil and gas facilities. At least 600,000 people were left without power, according to officials in Ukraine.

Mr. Zelensky had an audience in London on Wednesday with King Charles III and later met with Mr. Burnham, who promised accelerated military and economic support, including a new batch of air-defense missiles that have been effective against Russian drones.

“We will make sure that they’re in place as soon as possible,” Mr. Burnham said at the start of the meeting in No. 10 Downing Street.

Mark Rutte, the NATO secretary general, said at a news conference in Estonia: “Putin is trying to weaken our resolve and stop our support for Ukraine. But Russia is failing. It is failing on the battlefield in Ukraine. And we will not be intimidated.”Mr. Rutte, a former prime minister of the Netherlands, added, “Allies need to do more, especially in support of air defense” for Ukraine.

After Mr. Trump made a deal to import diesel fuel from Russia, a potential windfall for the Kremlin, his aides asked Ukraine to refrain from attacks on Russia’s oil refineries. Ukrainian officials interpreted what they heard from the Trump administration as a threat to halt intelligence aid.

Britain has a sophisticated intelligence operation and works with the United States, Canada, Australia and New Zealand as part of the Five Eyes alliance. However, America’s capabilities dwarf the others and would be particularly hard for Ukraine to lose, according to security officials.

“The precision, detail and timeliness of intelligence from the United States is noteworthy,” Robert B. Murrett, the former director of the U.S. National Geospatial Intelligence Agency, said in an interview on Wednesday. The United States would typically share information about targets inside Russia, said Mr. Murrett, a retired Navy vice admiral who now teaches at Syracuse University. Other European countries, including Poland and the Baltic States, also share “a fair amount of intelligence” with Ukraine, he added.

Mr. Murrett doubted that the U.S. intelligence community would pull its support from Ukraine. “The current leadership of the United States intelligence community and our allies is very anti-Russian,” he said. “That’s not to say the White House couldn’t overrule them,” he continued. “But the principal leadership of the U.S. intelligence community does not have a warm place in their hearts for Putin and his leadership in the Kremlin.”Thomas Withington, a military analyst at the Royal United Services Institute in London, said the capabilities of European intelligence agencies “are pretty good” but said the information they share can be fragmented, or duplicated, depending on how highly classified an individual nation deems it. That can also slow down intelligence being shared, he said.

“It’s not impossible for Europeans to replicate the signals intelligence and sharing that the U.S. does,” Mr. Withington said. “But obviously, having that additional backstop from the U.S., that additional capability, brings it more intelligence heft. So that’s why it’s very important.”Michael Schwirtz contributed reporting from London and Constant Méheut from Kyiv.

Michael D. Shear is the chief U.K. correspondent for The New York Times, covering British politics and culture and diplomacy around the world.

Lara Jakes, a Times reporter based in Rome, reports on conflict and diplomacy, with a focus on weapons and the wars in Ukraine and the Middle East. She has been a journalist for more than 30 years.