Rishi Sunak said he had made cooperation in tackling illegal immigration a priority at a meeting of EU leaders in Moldova.
The Prime Minister said he was “placing the fight against illegal immigration at the top of the international agenda” and that the UK was “taking the lead”.
The main theme of the European Political Community (EPC) summit was the war in Ukraine.
Before arriving for the negotiations at a castle near the Moldovan capital, Chisinau, he warned that Europe was “facing unprecedented threats on our border.”
The UK begins negotiations with Moldova, which has a population of 2.6 million, on a deal to return migrants arriving illegally in the UK.
However, only three Moldovan nationals arrived in the UK on a small boat last year, according to Home Office statistics.
A similar agreement with Georgia has now entered into force.
Around 300 Georgians arrived in the UK on small boats in 2022, and 31 in the first three months of this year.
The EPC – championed by French President Emmanuel Macron – was created in response to Russia’s invasion of Ukraine last year.
It includes 47 European countries, including European Union (EU) member states, the United Kingdom, Turkey, Norway and non-EU Balkan countries.
Ukrainian President Volodymyr Zelensky was also present at the summit, which was held less than 15 miles from his country’s border.
President Zelenksy, who called on NATO to make a clear commitment that Ukraine could join the Western defensive alliance, spoke with Mr Sunak.
Speaking at the summit, the prime minister acknowledged that Ukraine’s “rightful place” was within NATO, but gave no details on the path to membership.
He said the UK was working with its allies to ensure Ukraine had the “combat air capability needed to repel ongoing Russian aggression”.
In a tweet from Moldova, Mr Sunak said: “We have already reached migration deals with Albania, France and the EU to stop the boats.
“This global issue requires collaboration and the UK is taking the lead.”
The Times reported that the UK wants to increase its cooperation with Turkey and Bulgaria in efforts to tackle small boats crossing the Channel.
Earlier, Mr Sunak said: “Europe faces unprecedented threats on our borders. From Putin’s total disregard for the sovereignty of other countries to the rise of organized immigration crime on our continent.
“In every meeting, every summit, every international gathering like this, the security of our borders must be a priority.
“The UK will be at the heart of this international effort to stop the boats and defend our national security.”
Moldova, wedged between the EU and Ukraine, is a former Soviet state and, while its government turns to the West, pro-Russian separatists control the Transnistria region.
He welcomed thousands of Ukrainian refugees and the EPC promised him more support.
In March, Foreign Secretary James Cleverly announced £10 million of funding for Moldova’s energy sector, as well as economic and governance reforms.
Mr Macron said the new club of nations provides “a platform for political coordination” for countries inside and outside the EU.
But the EPC does not have dedicated institutions or staff. This raised questions about how the decisions would be implemented.