David Cameron has signalled he is willing to commit almost half a billion of the UK’s aid budget over the next five years in an effort to ease the flow of migrants and refugees to Europe.

The prime minister is prepared to offer £275m over the next two years to Turkey to help it cope with the scale of the refugee crisis it faces. The donation comes on top of £200m pledged until 2020 to help some African countries address the problems that have driven many migrants out of their home countries.

The extra support on offer for Turkey will be set out as EU leaders meet for talks after the conclusion of an international summit on the migrant crisis in Malta.

A No 10 source said: “We are sat in the western Mediterranean, which was the focus at the start of the crisis, but in recent months the focus has been on the eastern and the route from Turkey to Greece, where you see a lot more of the numbers of Syrian refugees as opposed to the route where we are sat today, which is more illegal migration. We remain really concerned about that route and the support that we should be providing to countries in the region.”

Underlining the scale of the challenge facing the Ankara government, the source said Turkish coastguards had picked up 63,000 migrants this year. “They have got 2 million refugees, it has cost them almost €7bn to date and we think there is much more the EU can be doing there, alongside the work we have been doing in Jordan and Lebanon,” the source said.

“So we are going to say that we would be willing to provide up to £275mn over the next two years as part of a stepped-up contribution from European countries to help Turkey.”

The money could be used to fund new refugee camps and provisions for those left homeless by the conflict in Syria. It would also help Syrian refugees stay closer to their homeland, which has been ravaged by the civil war and the rise of Islamic State.

“The more you contain the problem in and around Syria so that if and when you find a solution on Syria it’s easier for people to go home,” the source said. The funding could also be used to tackle the “small-scale industry” selling rafts to desperate migrants trying to leave Turkey for Europe. The money for Turkey and Africa would come out of the 0.7% of gross national income committed to overseas aid.

The No 10 source said: “It is us looking at how we focus our spending on humanitarian assistance, but also dealing with an issue that has repercussions for us in Britain. So we are investing our aid money upstream and overseas to better manage the problem arriving at our shores.”

As well as the extra funding for African states, the government will also urge them to do more to accept the return of migrants who have tried to enter the EU illegally. “The prime minister will be emphasising to the Africans that they have got to work with us on returns. We welcome the cooperation they have shown so far, but it’s very important that we develop the situation where we are able to return illegal migrants who arrive in the Mediterranean,” the source said.

The extra support for some African countries and Turkey came after Cameron pledged that the UK would play a “huge and historic role” in helping to tackle the crisis, including increasing efforts to “smash” gangs of human traffickers.

The prime minister said it was the biggest problem facing Europe today, with a movement of people greater than since the end of the second world war.