Prime Minister David Cameron said Sunday he will double Britain's drone fleet and warned about the prospect of Britain's exit from the European Union.

Cameron said the government would purchase 20 "Protector" drones and spoke confidently about the prospects of a parliamentary vote to join air strikes targeting the Islamic State group in Syria.

He said Britain's fleet of 10 "Reapers" would be replaced by 20 new models as part of the country's 2015 Defence Review.

"This will be combined with increasing the capacity of our special forces, so that the country remains ready to address any threats to our nation's security," he said in a statement.

The Protectors, to be imported from the United States, can be used as surveillance craft but can also be armed, the Ministry of Defence said Sunday.

More drones will "keep us safe and... give us the intelligence and information and potentially give us the capacity to hit people who are potentially planning to hit us", he told the Sunday Telegraph.

Cameron announced last month that a Royal Air Force drone strike killed two British Islamic State jihadists and another militant in the group's stronghold of Raqa in August.

In his first newspaper interview since winning a May general election, Cameron also spoke about his negotiations with fellow European leaders about EU reforms.

He described the talks as "bloody hard work".

"I think the right answer is for Britain to stay in a reformed European Union," he said.

"But I've always said if we don't get those things that we are asking for I rule nothing out and I am very serious about that," he added.

Cameron has promised to hold a referendum on whether Britain should stay in or leave the EU by 2017 at the latest but has said he wants to renegotiate its terms of membership.

He has said he will push for broad changes including protecting sovereignty, opting out of the EU's commitment to "ever closer union", limiting access to benefits for migrants, and boosting competitiveness.

The interview comes ahead of the annual conference of Cameron's Conservative Party on Sunday in Manchester, where a major rally by trade unionists and anti-austerity campaigners is also expected.