Zelensky thanks Britain during his second overseas trip since Russia’s invasion.

LONDON — President Volodymyr Zelensky of Ukraine made a surprise visit to Britain on Wednesday morning, welcomed with a hug at Stansted Airport by Prime Minister Rishi Sunak, who announced plans to expand British military support for Ukraine to include training its pilots in NATO fighter jets.

In a speech to Parliament, Mr. Zelensky, dressed in his familiar drab green sweatshirt and speaking in English, thanked Britain for its military support “on behalf of our warriors who are now in the trenches under enemy artillery fire.”

“London has stood with Kyiv since Day 1,” he said, adding: “We know that freedom will win.”

Mr. Zelensky is scheduled to meet with King Charles III at Buckingham Palace later in the day. Earlier on Wednesday, applause rang out for Mr. Zelensky as he walked into 10 Downing Street with Mr. Sunak, after the two leaders shook hands and waved to reporters and onlookers.

The visit by Mr. Zelensky, announced by Downing Street just hours before his arrival, is only the second trip the Ukrainian leader has made outside his country since Russia’s invasion last February. In December, he traveled to Washington for a White House meeting with President Biden and an address to Congress, and also stopped in Poland.

The trip underscores Britain’s role as one of the largest suppliers of weapons to Ukraine — the United States is the largest — and the robust public support for Mr. Zelensky by Mr. Sunak and his two predecessors, Liz Truss and Boris Johnson. Both Mr. Sunak and Mr. Johnson traveled to Ukraine to meet with Mr. Zelensky, although Ms. Truss, whose tenure as the British leader lasted a little over six weeks, never made the trip as prime minister.

Mr. Zelensky is expected to travel to Brussels on Thursday for meetings with European Union leaders.

Britain is sending Ukraine 14 Challenger 2 battle tanks, as well as artillery and thousands of rounds of ammunition, a pledge that it made several weeks ago, in part to prod a reluctant Germany and the United States to follow suit. Those two governments have now also promised to send battle tanks, which military analysts say will fortify Ukraine’s ability to retake Russian-occupied territory in the face of an expected Russian spring offensive.

So far, Mr. Sunak, like the Biden administration, has resisted calls to send fighter jets to Ukraine for fear that they would escalate the confrontation with Russia. But Mr. Johnson, on a visit to Washington last week, called for Britain and the West to do whatever it takes — including supplying jets — to enable Ukraine to push back Russian troops.

It was not immediately clear if Britain’s plan to train pilots signaled a change in its stance on jets, though that would be in keeping with the British government’s strategy of being in the vanguard of Western military support for Ukraine.

Britain has already trained 10,000 Ukrainian troops, including soldiers who arrived last week to learn how to operate the Challenger tanks. In addition to pilots, Britain said it would begin to train Ukrainian marines.

In a statement, Mr. Sunak said that expanding the training to marines and fighter jet pilots “underlines our commitment to not just provide military equipment for the short term, but a long-term pledge to stand shoulder-to-shoulder with Ukraine for years to come.”

The British government also announced new sanctions targeting the Russian military and loyalists of President Vladimir V. Putin, with the aim of “undermining his war machine to help Ukraine prevail,” the British foreign secretary, James Cleverly,