Britain's Prime Minister David Cameron pledged support on Friday for Afghanistan's newly sworn-in president and the country's new unity government, saying during a surprise visit to Kabul that Britain is committed to helping Afghans build a more secure and prosperous future.

Cameron was the first of the world leaders to meet Ashraf Ghani Ahmadzai, Afghanistan's second elected president, since his inauguration on Monday. The two had a meeting in Kabul on Friday morning and later held a joint news conference.

"Britain has paid a heavy price for helping to bring stability to this country," Cameron said, paying tribute to the 453 British servicemen and women who died while serving in Afghanistan.

"An Afghanistan free from al-Qaida is in our national interest - as well as Afghanistan's," he said. "And now, 13 long years later, Afghanistan can - and must - deliver its own security."

Cameron arrived a day after visiting British pilots in Cyprus, who are taking part in airstrikes on Islamic State group targets in Iraq. British warplanes have been conducting combat missions over Iraq since Saturday, after Britain joined the US-led coalition of nations that are launching airstrikes against the militants.

Ghani Ahmadzai thanked the British for their sacrifices in Afghanistan, especially the families who lost loved ones in the war. "They stood shoulder to shoulder with us and we will remember," he said.

Ghani Ahmadzai's inauguration this week marked the start of a new era for his country, with a national unity government poised to confront a resilient Taliban insurgency.

A day after he was sworn in, his administration signed a security agreement allowing the United States to keep about 9,800 troops in the country to train and assist Afghan national security forces.