"Kyiv has captured a part of my heart," he added.

The clandestine nature of the trip points to the dangers of America's commander-in-chief visiting Ukraine amid continuing bombardment, as Russia continued firing missiles into the country hoping to break an impasse as the war enters its second year. Air raid sirens sounded while Biden was on the ground.

It is the first time in modern history that a U.S. president has entered a war zone where there is not an active American military presence. Ukrainian police and military had a strong presence and much of the central part of Kyiv was shut down, paralyzing traffic nearby.

Biden's appearance in Ukraine is a strong statement that the U.S. stands with Zelenskyy despite growing pressure at home to downsize American aid.

"Joseph Biden, welcome to Kyiv!" Zelenskyy posted on the social media site Telegram along with a picture of the two leaders. "Your visit is an extremely important sign of support for all Ukrainians."

Biden visited the Mariinsky Palace, where he signed a guest book and made brief remarks alongside Zelenskyy.

Biden insisted that the U.S. will remain steadfast in its support of Ukraine.

"For all the disagreement we have in our Congress on some issues, there is significant agreement on support for Ukraine,” Biden said.

He announced an additional aid package worth $500 million, which will include more military equipment and weapons like anti-tank Javelin missiles.

Zelenskyy said that when Russia invaded, the U.S. was the first to call him in support.

Biden also recalled speaking to Zelenskyy — and the promise he made to help "rally the world" to Ukraine's cause.

“Russian planes were in the air and tanks were rolling across your border," Biden said. "You told me you could hear explosions in the background, I’ll never forget that. And the world was about to change. I remember it vividly."

The two leaders greeted each other with a sense of familiarity.

“Thank you for coming,” Zelenskyy said, according to a press pool report.

“More importantly, how are the children?” Biden said. He added, “It’s amazing to see you.”

The two leaders visited St. Michael’s Golden-Domed Monastery and then walked to the nearby Wall of Remembrance, which honors those who have died in the war.

Biden also visited the U.S. embassy in Kyiv.

The trip came with risks. Other presidents have visited war zones. Barack Obama traveled to Afghanistan in 2014 and George W. Bush visited American troops in Iraq in 2003. As was the case with Biden, both traveled under strict secrecy.

White House officials had been adamant that Warsaw, Poland, would be Biden’s only stop. On Friday, a White House spokesman answered with a single word when asked if Biden intended to cross the border into neighboring Ukraine: “No.”

Now, the adversary is Russia. If Russian President Vladimir Putin's military were to intentionally target Biden, or even inadvertently harm the president by way of an errant missile, the U.S. would be obligated to retaliate. That could potentially escalate a regional war into a direct conflict between two nuclear-armed countries.

Still, ahead of the trip, some analysts were hopeful that Poland would not be Biden's only stop. A trip to Ukraine "would be a powerful demonstration of support and signal a robust change in policy — a more forward-leaning and fulsome approach to Ukrainian support," said Alexander Vindman, former director of European affairs in the Donald Trump White House's National Security Council.

Biden has invested considerable political, military and financial capital in the war, keeping overmatched Ukraine viable against larger Russian forces. In addition to American assistance, he has also pressed the rest of the NATO alliance to shore up Ukraine, arguing it is imperative for the democratic world to counter Russian aggression for two reasons. One is to deter Putin from widening the war into NATO countries; another to discourage China's authoritarian government from launching its own attacks on smaller nations such as Taiwan.

The U.S. has also begun to sound warnings that the Chinese are providing military assistance to the Russian effort and cautioned the country against providing lethal aid.

China has provided some broader help to Russia, including parroting Russian disinformation and promoting Russia's false pretexts for the war. But U.S. officials say the Chinese are now providing more tangible assistance for Russian troops in Ukraine. The Chinese have in the past denied providing military assistance.

Biden and Zelenskyy met in December in Washington, D.C., when the Ukrainian president traveled to the U.S. capital and delivered an address to a joint session of Congress. Zelenskyy has only sparingly left his nation since the invasion.

With fighting picking up, Zelenskyy has been pressing Washington for more advanced fighter jets. So far, Biden has refused out of concern that the F-16 planes might escalate the conflict.

In January, Biden announced the U.S. would send 31 Abrams tanks, which he was initially reluctant to do. The U.S. will also provide training for the tanks.

Biden is also facing cross-pressures from conservative Republicans, who believe the U.S. has already given enough money and arms to Ukraine and should now press for a peace deal.

The amount of American military assistance to Ukraine is nearing $30 billion. Polling shows that American support for Ukraine remains strong, though less so as time passes. In May 2022 — three months into the war — 60% of Americans favored supplying weapons to Ukraine. As of January, that number had slipped to 48%, according to an Associated Press-NORC Center for Public Affairs Research poll released last week. The percentage of adults who favored sending taxpayer money directly to Ukraine dropped seven percentage points in that time frame — from 44% to 37%.

The war has stretched on longer than most in the West expected, with initial estimates that Ukraine would be able to hang on only for a few weeks before the Russians seized Kyiv, ousted Zelenskyy and absorbed the country into its orbit. Instead, Ukraine proved to be a more resilient opponent than anticipated.

Peter Nicholas reported from Warsaw and Cat Corrigan and Susan Kroll reported from Kyiv.