US president Joe Biden made a surprise visit to Kyiv on Monday in a dramatic show of American commitment to Ukraine ahead of the first anniversary of Russia’s full-scale invasion.

His stopover, shrouded in secrecy owing to security concerns, comes at a pivotal moment in the war as Ukraine gears up for a counteroffensive and calls on its allies to speed up the delivery of billions of dollars in western weapons. Russia has also increased attacks and preparations for a larger offensive, though progress has been halting so far.

“I thought it was critical that there not be any doubt, none whatsoever, about US support for Ukraine in the war,” Biden said during joint remarks with Ukrainian president Volodymyr Zelenskyy in Mariinsky Palace, the official residence of the Ukrainian president. “I’m here to show our unwavering support for the nation’s independence, sovereignty and territorial integrity.”

Zelenskyy thanked Biden for coming at “the most difficult time” for Ukraine. “This conversation brings us closer to victory,” he said.

While in Kyiv, Biden announced $500mn in additional security assistance, including artillery ammunition, more javelin missiles and howitzers. The package did not appear to include new capabilities such as fighter jets or longer-range missiles Kyiv has long sought. The US has announced about $30bn in lethal assistance since Russia’s full-scale invasion of Ukraine on February 24 2022.

Zelenskyy said he and Biden discussed the future provision of longer-range missiles that Ukraine had not yet received.

Biden said the US would impose more sanctions against Russian elites and companies assisting Russia’s war effort.

The visit is a symbol of American resolve, recalling other clandestine wartime visits of US presidents to Iraq and Afghanistan. Monday’s trip to Kyiv was all the more daring given the lack of US troops on the ground to help support it.

The US notified Russia that Biden would visit Kyiv a few hours before his departure “for deconfliction purposes,” US national security adviser Jake Sullivan said. He declined to describe the precise nature of the message or how Russia responded, citing “the sensitive nature of those communications”. Moscow confirmed having received advance notice of Biden’s trip.

Videos on social media showed Biden walking with Zelenskyy in Kyiv’s St Michael’s Square as air raid sirens blared in the background and sounded throughout much of the country. With the backdrop of the gold-domed St Michael’s Cathedral, Biden laid a wreath at a memorial to fallen Ukrainian soldiers in the centre of the square.

While in Kyiv Monday, Biden vowed to stand with Ukraine for “as long as it takes”. One of his goals was to come to an understanding with Zelenskyy about what “it” was.

The two presidents sought to map out the coming year, seeking “a common understanding” of what their objectives were, “where Ukraine is trying to get [to] and how the United States can most effectively support them alongside our allies and partners,” Sullivan said.

The stark images of Biden and Zelenskyy walking together in downtown Kyiv come as some Republican lawmakers, including Speaker of the House Kevin McCarthy, have questioned the US’s ability to sustain long-term assistance to Ukraine.

In the coming weeks, Biden is expected to make an additional request for funds to Congress to provide more lethal assistance to Ukraine, which will give some indication of the Republican-controlled House’s stand on the issues and how the question of aid for Ukraine might play in the coming 2024 presidential elections.

Later on Monday, Biden is expected to travel to Poland for a two-day trip, during which he will make a centrepiece speech on the war and meet Polish president Andrzej Duda as well as the leaders of the so-called Bucharest Nine group of eastern European countries.

On Tuesday, Russia’s president Vladimir Putin was planning to address the war and its impact on Russian society in a delayed state-of-the-nation address, the Kremlin said.

Biden left Washington with a small group of aides at 4.15am on Sunday and arrived in Kyiv on Monday at 8am local time after taking an overnight train. Biden left the Ukrainian capital later that afternoon.

Planning for the trip was under way “over a period of months,” involving a small circle in the US and Ukraine, deputy national security adviser Jon Finer said. Biden made the final decision to go on Friday after receiving an Oval Office briefing from several members of his security cabinet.

Many of Biden’s western counterparts, such as UK prime minister Rishi Sunak, French president Emmanuel Macron and German chancellor Olaf Scholz, have already visited Kyiv. Biden’s visit follows trips by others in his administration including Blinken and Sullivan.

His visit was the first by an American president in nearly 15 years. The last to go to Kyiv was George W Bush in 2008, when he was pressing to bring Ukraine into Nato.

Biden’s aides said he was excited to make the trip, and placed great importance on being in Ukraine in this moment.

“One year later, Kyiv stands and Ukraine stands, democracy stands,” Biden said. “The Americans stand with you and the world stands with you. Kyiv has captured a part of my heart.”

Additional reporting by Max Seddon in Riga