Biden's upcoming European trip is meant to boost NATO against Russia as the war in Ukraine drags on

President Joe Biden will head to Europe at week's end for a three-country trip intended to bolster the international coalition against Russian aggression as the war in Ukraine extends well into its second year.

The main focus of Biden's five-day visit will be the annual NATO summit, held this year in Vilnius, Lithuania. Also planned are stops in Helsinki, Finland, to commemorate the Nordic country's entrance into the 31-nation military alliance in April, and Britain, the White House announced Sunday.

Biden will begin his trip next Sunday in London and will meet with King Charles III at Windsor Castle the next day, according to Buckingham Palace. The president did not attend Charles's coronation in May, sending first lady Jill Biden to represent the United States. In June, Biden hosted British Prime Minister Rishi Sunak at the White House, where the two leaders pledged continued cooperation in defending Ukraine.

Sunak's office said he looked forward to welcoming Biden and that their meeting would build on earlier visits.

The NATO meeting comes at the latest critical point in the war. Ukraine's president, Volodymyr Zelenskyy, says counteroffensive and defensive actions against Russian forces are underway as Ukrainian troops start to recapture territory in the southeastern part of the country, according to its military leaders.

Jens Stoltenberg, NATO's secretary-general, visited the White House on June 13, where he and Biden made clear that the Western alliance was united in defending Ukraine. Biden said during that meeting that he and other NATO leaders will work to ensure that each member country spends the requisite 2% of its gross domestic product on defence.

"The NATO allies have never been more united. We both worked like hell to make sure that happened. And so far, so good," Biden said as he sat alongside Stoltenberg, who is expected to extend his term for another year. "We see our joint strength in modernising the relationship within NATO, as well as providing assistance to defence capabilities to Ukraine.

When Finland joined NATO in April, it effectively doubled Russia's border with the world's biggest security alliance. Biden has highlighted the strengthened NATO alliance as a signal of Moscow's declining influence.

Sweden is also seeking entry into NATO, although alliance members Turkey and Hungary have yet to endorse the move. Biden will host Sweden's prime minister, Ulf Kristersson, at the White House on Wednesday in a show of solidarity as the United States presses for the Nordic nation's entry into NATO.

Turkey's president, Recep Tayyip Erdogan, has said Sweden is too lax on terrorist groups and security threats. Stoltenberg has said Sweden has met its obligations for membership through toughening anti-terrorist laws and other measures.

Hungary's reasons for opposing Sweden have been less defined, complaining about Sweden's criticism of democratic backsliding and the erosion of the rule of law. Hungary, while providing humanitarian aid to Ukraine, has also sought to balance its relations between NATO and Russia. Budapest is heavily reliant on Russia for its energy requirements.

All nations in the alliance have to ratify the entry of a new member country.

The White House has stressed that Sweden has fulfilled its commitment to join NATO and has urged that it join the alliance expeditiously.