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

On a trip shrouded by secrecy for security reasons after months of planning, Biden announced $500mn in new military aid.

“I’m here to show our unwavering support for the nation’s independence, sovereignty and territorial integrity,” Biden said, vowing to stand with Ukraine for “as long as it takes”.

The stopover came at a pivotal moment in the war, as western nations discuss how far to go in arming Kyiv and as the 80-year old president gears up for a possible re-election campaign.

Ukraine is calling for the speedy delivery of billions of dollars in western weapons, while Russia has increased attacks and is preparing a larger offensive, although its progress has been halting so far.

President Volodymyr Zelenskyy thanked Biden for coming at “the most difficult time” for his country. “This conversation brings us closer to victory,” Zelenskyy said at a joint appearance at Mariinsky Palace, his official residence.

Jake Sullivan, US national security adviser, said the two presidents sought to map out the coming year, including “a common understanding” of objectives “and how the United States can most effectively support them alongside our allies and partners”.

The visit resembled previous US presidents’ clandestine wartime trips to Iraq and Afghanistan — but was more daring since US troops were not on the ground to help provide support. Sullivan said Washington alerted Russia before Biden’s arrival “for deconfliction purposes”.

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

The images of the two men come as some Republican lawmakers, including House of Representatives Speaker Kevin McCarthy, have questioned the US’s ability to sustain long-term assistance to Ukraine.

Biden is expected in coming weeks to request more funds from Congress to provide lethal assistance to Ukraine. The move could force the new Republican leadership of the House to clarify its stance on the issue, as well as indicating how it may play in the 2024 election.

The $500mn of aid Biden announced on Monday includes artillery ammunition, more javelin missiles and howitzers, adding to about $30bn of US lethal assistance since the invasion on February 24 last year.

The package did not appear to include capabilities long sought by Kyiv, such as fighter jets or longer-range missiles, although Zelenskyy said the two presidents had discussed the future provision of such missiles.

“One year later, Kyiv stands and Ukraine stands, democracy stands,” Biden said, in remarks referring to last year’s invasion. “The Americans stand with you and the world stands with you.”

He added that the US would impose new sanctions against Russian elites and companies assisting President Vladimir Putin’s war effort.

The trip to Ukraine is the first by an American president in nearly 15 years, but follows a series of visits by other western leaders and members of his own administration.

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.

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.

The US president left Kyiv on Monday afternoon for Poland, where he is due on Tuesday to give a set piece speech on the conflict — on the same day that Putin is expected to make his own state-of-the-nation address on the war.

Republican Senator Lindsey Graham, who has been hawkish on support for Ukraine, said he was “pleased” Biden had made the trip to Ukraine. But he urged the administration to declare Russia a state sponsor of terror, a move US officials oppose for fear it could hinder aid delivery to Ukraine, as well as deliver F-16 jets to Kyiv.

“This was the right signal to send at the right time,” Graham said. “However, for the visits and statements to matter to their fullest extent, decisive action must immediately follow.”

Additional reporting by Max Seddon in Riga