The UK will remain a “reliable” partner for the EU in helping to tackle the Mediterranean migration crisis even after Brexit, Theresa May is to tell her fellow European leaders at a summit in Malta on Friday.

The prime minister will use the meeting to reassure the remaining 27 EU members that Britain will not step back from such international obligations, Downing Street said, while also urging them to spend more on defence in the era of Donald Trump’s scepticism about Nato.

The summit, held at the Grand Master’s Palace in Valletta, will be May’s first encounter with the other EU nations since a meeting in Bratislava in December, which at one point saw May pictured standing alone as other leaders chatted around her.

May is expected to stay only for the morning session and working lunch, which is focused on how to deal with the number of migrants and refugees seeking to enter Europe via the Mediterranean and Libya, and the human cost, both in terms of assistance and the large numbers who drown trying to cross.

The agenda is directed at trying to stop people smugglers, and seeking ways the EU can better cooperate with the authorities in Libya, a major exit point for those seeking to enter Europe.

A Downing Street statement released in advance of the meeting said May would “stress that migration has been one of her political priorities during her time in government – and remains so”.

It added: “She will say that the UK has played a central part in tackling this crisis – and will remain a reliable partner.”

May is also expected to have some bilateral chats with fellow EU leaders, though details of any plans have yet to be released.

The prime minister’s spokeswoman said she wanted to keep strong EU links after Brexit. “We are very clear we want to see a strong and successful EU, now and into the future, that we can have a mature and constructive partnership,” she said.

The summit comes amid a busy period of international meetings for May. Last week she made a high-profile and controversial visit to Washington to see Trump, before holding talks in Ankara with Turkey’s president, Recep Tayyip Erdoğan.

On Monday she is due to host the Israeli prime minister, Benjamin Netanyahu, at Downing Street.

Her office said she would brief EU leaders over her US visit, and what May said was a guarantee from Trump to be completely supportive over Nato. She will also stress the need for other Nato members to meet the commitment of spending 2% of their GDP on defence, “so that the burden is more fairly shared”.

May will miss the afternoon session of the Valletta summit, at which the remaining 27 EU leaders will resume discussion of how Brexit can be handled, and preparations for next month’s 60th anniversary of the Treaty of Rome, which set up the EU’s precursor, the European Economic Community.

It comes a day after the UK government set out its longest statement of intent yet on Brexit, with the publication of a 77-page parliamentary white paper about the plans for the process.

In a letter this week to EU leaders, Donald Tusk, president of the European council, who will chair the talks, said the anniversary would be a chance to “strongly reiterate these two basic, yet forgotten, truths: firstly, we have united in order to avoid another historic catastrophe, and secondly, that the times of European unity have been the best times in all of Europe’s centuries-long history”.

He added: “It must be made crystal clear that the disintegration of the European Union will not lead to the restoration of some mythical, full sovereignty of its member states, but to their real and factual dependence on the great superpowers: the United States, Russia and China. Only together can we be fully independent.”