US President Joe Biden began his three-nation Europe trip after arriving in London on Sunday.

He is set to meet with British Prime Minister Rishi Sunak at 10 Downing Street on Monday, after which he will visit King Charles at Windsor Castle.

The talks with King Charles are expected to include climate initiatives.

On Monday night, Biden will travel to Lithuanian capital Vilnius ahead of talks with NATO leaders slated for June 11 and 12.

Expected to be on the agenda are NATO's support for Kyiv, as well as Sweden's bid to join the alliance, which has so far been blocked by Turkey.

The Vilnius summit also comes after the Biden administration greenlit the delivery of cluster munitions to Ukraine, with some NATO allies such as the UK and Spain expressing concern about the move.

After his trip to Lithuania, Biden will head to Finland, which is the newest member of the NATO alliance.

Ukraine not 'ready' for NATO membership, Biden says

The US president had cast doubt on the likelihood of Ukraine being offered NATO membership in comments to the CNN broadcaster.

"I don't think it's ready for membership in NATO," Biden told CNN in an interview aired Sunday.

Biden said that while he hoped that NATO leaders would "lay out a rational path for Ukraine to be able to qualify to be able to get into NATO."

He said that in order to join the military alliance countries must "meet all the qualifications, from democratization to a whole range of other issues." He added that bringing Ukraine into the alliance now would mean "war with Russia."

"I don't think there is unanimity in NATO about whether or not to bring Ukraine into the NATO family now, at this moment, in the middle of a war," he said.

Biden added that it will "take a while" for Ukraine to become a member of NATO.

However, Biden said that Washington could provide Kyiv with support similar to that it gives to Israel until Ukraine joins NATO.

"The United States would be ready to provide...security a la the security we provide for Israel." He said this involves "providing the weaponry they need, the capacity to defend themselves."

According to a recent report by the US Congressional Research Service, no other country in the world has received more US support than Israel since World War II.

Biden calls for Sweden in NATO during Erdogan call

Also on Sunday, the White House said that Biden expressed a desire for Sweden to join NATO "as soon as possible" in a phone call with Turkish President Tayyip Erdogan.

Sweden's NATO bid requires unanimous approval from all member states, including Turkey. On Thursday, Stockholm failed to convince Ankara to ratify Swedish accession to NATO in a foreign minister-level meeting.

Erdogan told Biden that Sweden had taken steps in the right direction for Turkey to support the bid, the Turkish presidency's communications directorate said in a separate statement on Sunday.

However, it said that the steps were not useful as Kurdistan Workers Party (PKK) supporters continued to hold demonstrations in Sweden.