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(AFP) - US President Joe Biden was in Britain on Monday for a brief visit to his key ally during which he will meet King Charles III and Prime Minister Rishi Sunak before going on to a NATO summit in Lithuania.

Ahead of the visit, the White House said Biden would seek to "further strengthen the close relationship" with Britain.

The king will host Biden at Windsor Castle, a royal residence west of London, where they are due to discuss climate issues, long a subject close to Charles' heart.

It will be their first meeting since the monarch's coronation in early May.

The president did not attend, sending First Lady Jill Biden instead, but the Bidens were at the funeral of Queen Elizabeth II in September.

Earlier in the day, Biden will meet Sunak for their fifth meeting in recent months.

Downing Street has said the president's visit "reflects the strong relationship" between the two countries.

Sunak travelled to the United States last month for a two-day visit where the leaders discussed Ukraine, AI regulation and post-Brexit ties.

They also agreed to a new economic partnership, the "Atlantic Declaration".

Biden said the United States has "no closer ally" than Britain.

But Sunak came away empty-handed on the UK's ambitions for a post-Brexit free-trade agreement with Washington.

The two countries are key allies of Ukraine and Russia's invasion of its pro-Western neighbour is likely to top the agenda on Monday, after Biden pledged to supply Kyiv with controversial cluster munitions.

- 'Compare notes' on Ukraine -

The move raised concerns from rights groups due to the danger unexploded bomblets pose to the civilian population and caused unease among US allies.

Biden said the decision to send the weapons was "very difficult" but Ukrainian forces conducting a counteroffensive against invading Russian troops were "running out of ammunition".

More than 120 countries have signed an agreement banning the production, storage, sale and use of cluster munitions and Sunak on Saturday highlighted that Britain was among them.

Sunak stressed, however, that Britain -- which has supplied Kyiv with heavy battle tanks and long-range weapons -- will continue to support Ukraine.

The talks will be an opportunity for Biden and Sunak to "compare notes" on Ukraine ahead of a meeting with fellow NATO leaders in Lithuania, Amanda Sloat, National Security Council senior director for Europe, said in a briefing on Friday.

NATO allies meet in the capital, Vilnius, on Tuesday and Wednesday for a summit where Ukraine is hoping for a clear signal that it could one day join the US-led alliance.

From Lithuania, Biden will travel to Finland, NATO's newest member, for a US-Nordic Leaders Summit.

Northern Ireland may also be on the agenda for Sunak and Biden as it is "certainly an issue that the president regularly talks about with his British counterpart", Sloat said.

Biden briefly visited Northern Ireland in April to mark 25 years since the signing of the Good Friday Agreement, which ended three decades of violence in the UK province, under US mediation.

Biden, proud of his Irish roots, has criticised the UK for its stance on post-Brexit trading rules in Northern Ireland, warning it could undermine the hard-fought peace.

© Agence France-Presse