UK Prime Minister Keir Starmer on Thursday vowed to rebuild trust with European allies damaged by Brexit as he hosted dozens of leaders at a summit where they reaffirmed backing for Ukraine and pledged to tackle irregular migration.

Starmer, premier for two weeks, told more than 45 European leaders the UK would be a "friend and partner" as he started work to improve ties hit by the country's tortuous departure from the European Union under his Conservative predecessors.

"We want to work with all of you to reset relationships, rediscover our common interest and renew the bonds of trust and friendship that brighten the fabric of European Life," the 61-year-old Labour leader told them as he opened the European Political Community (EPC) meeting.

Europe is grappling with challenges ranging from supporting Kyiv after Russia's full-scale assault to breaking up people-smuggling gangs contributing to record levels of irregular migration.

In the background hangs the increasing possibility of Donald Trump regaining the White House in November, with fears over his commitment to the Western military alliance Nato.

Trump's newly elected running mate JD Vance is an outspoken critic of military aid to Ukraine and has pushed for negotiations with Russia to end the two-year-plus war.

"It's very important to be here, for Ukraine, especially this tough period, for us during the war," Ukrainian President Volodymyr Zelensky said at Blenheim Palace, near Oxford, west of London.

"For us it's very important to maintain unity in Europe because always this unity leads to strong decisions," added Zelensky before discussions with Starmer and a meeting with head of state King Charles.

While at the summit, the Ukrainian leader signed long-term security cooperation agreements with the Czech Republic and Slovenia.

Nato Secretary-General Jens Stoltenberg, attending an EPC summit for the first time, reaffirmed the alliance's support for Ukraine.

In a departure from the often fractious relations that characterised the run-up to Brexit, Starmer, who led Labour to a landslide election win over the Tories on July 4, told European counterparts that his government would be "a friend and a partner ready to work with you."

He called irregular migration a "crisis" affecting every country attending. More than 380,000 undocumented migrants crossed the EU's borders last year, and tens of thousands made their way to Britain.

Starmer called for a sharing of resources, intelligence and tactics to "shut down the smuggling routes and smash the gangs," behind migrant Channel crossings from northern France in small boats.

It was a problem that dogged the Tory administrations under Boris Johnson, Liz Truss and Rishi Sunak.

Cooperation was "the most efficient" way to tackle the problem, French President Emmanuel Macron said, the morning after one migrant died and 71 others were rescued off the French coast.

Starmer has conversations pencilled in with several European leaders, including Macron, as he fleshes out what he wants the new relationship with the EU to look like.

The former human rights lawyer and chief state prosecutor supported Britain remaining in the EU during the 2016 Brexit referendum and was once Labour's Brexit spokesman.

He has ruled out rejoining the European single market, customs union or freedom of movement.

But he does want to negotiate a new security pact with the bloc and a veterinary agreement to ease border checks on agricultural foods, as well as an improved trading deal.

Starmer was quick out of the blocks to congratulate Ursula von der Leyen on securing another term as European Commission president. He eagerly anticipated "working closely" with the EU chief, he said.

Irish Premier Simon Harris described Labour's reset with Europe as a "gamechanger" while European Council chief Charles Michel said he hoped that a "new chapter" would be opened with the UK.

One high-ranking EU official said there were "good vibes" in the early contacts with the new Labour government, but warned that the EU was not looking to "reopen" the Brexit deal with Britain.

Blenheim Palace is the birthplace of Britain's World War II leader Winston Churchill. The Treaty of London, which signed into existence the Council of Europe in 1949, was on display, adding further symbolism to the proceedings. (AFP)