British Prime Minister Rishi Sunak has visited Kyiv for talks with Ukrainian President Volodymyr Zelenskyy, and reiterated his country's unwavering support for Ukraine.

Sunak announced on Friday that Britain will provide 2.5 billion pounds, or about 3.2 billion dollars, in military aid to Ukraine next financial year, including long-range missiles and thousands of drones.

Sunak also announced he signed an agreement with Ukraine to provide long-term security support to the country, based on the Group of Seven declaration in July last year. Officials say Britain is the first G7 country to sign such a pact.

Zelenskyy welcomed the move, saying, "Today is the day when the history of Europe has changed." He said their countries' shared challenge is to ensure that attacks on Ukraine will never be repeated.

The United States, Ukraine's biggest backer, faces a stalemate in Congress over an emergency spending bill to continue military assistance to Ukraine.

Leaders of the European Union have also failed to agree on financial aid to Kyiv, held up by Hungary's opposition.

Sunak stressed Britain was taking the initiative to provide support for Ukraine to show that his country and the free world will continue to stand with Kyiv.

Responding to the news on Friday, former Russian president and the deputy chair of Russia's Security Council, Dmitry Medvedev, issued a warning to Britain on social media.

He said, "I hope our eternal enemies, the insolent British, realize that a deployment of their official military contingent in Ukraine would mean a declaration of war on our country."

That followed a warning from Medvedev on Thursday that Ukraine's attempts to destroy Russia's missile launchers with Western missiles could potentially trigger a nuclear response.