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Gen David Petraeus, US commander of Nato troops in Afghanistan, has handed responsibility for the military campaign there to his successor, Gen John Allen.

Gen Petraeus, who is returning to the US to run the CIA, oversaw last year's surge of US troops to Afghanistan.

The transition comes hours after a close aide to Afghan President Hamid Karzai died in an attack in Kabul.

Earlier on Sunday, Nato handed control of Bamiyan province to local forces.

It is the first of seven areas to be passed to Afghan security forces under a plan announced by President Karzai in March.

The handover from Nato to Afghan forces is seen as a critical step in a transition of power before foreign troops end combat operations in 2014.

At the change of command ceremony in the Afghan capital, Kabul, David Petraeus, expressed his "profound and lasting" gratitude to Afghan and Isaf troops for their service.

He also addressed the people of Afghanistan saying: "You have shown enemies of Afghanistan that you are willing and able to resist a campaign of violence and intimidation."

His successor reinforced his commitment to support the security transition to Afghan forces.

"'There will be tough days ahead and I have no illusions about the challenges we will face together," Gen John Allen told those assembled at the ceremony.

The US military's top officer, Adm Mike Mullen, was also present at the ceremony. Gen Petraeus, who has served with the US Army for 37 years, is credited with turning around the war in Iraq after former President George W Bush tasked him with managing a surge of troops there in 2007.

Gen Petraeus took charge in Afghanistan last year at a critical moment and managed the deployment of more than 30,000 US troops to fight the Taliban. US troops are set to begin the first phase of their withdrawal in the coming months.

The ceremony was conducted as Nato announced the deaths of three coalition soldiers in eastern Afghanistan after a bomb attack.

And the killing of a key presidential aide to President Karzai in the Afghan capital also highlights the challenges that lie ahead, says the ´óÏó´«Ã½'s Sanjoy Majumder in Kabul. Less than a week earlier, Mr Karzai's half-brother, Ahmed Wali Karzai, was assassinated in Kandahar.

Jan Mohammad Khan was killed in an attack on his home in Kabul on Sunday.

Lawmaker Hasham Watanwal also died in the attack, said police, as gunmen stormed the house in the western Karti Char district of the city.

A shoot-out with security forces in the area continued for some time afterwards, police said. One report said the attackers had been wearing suicide vests.

The question many are asking, according to our correspondent in Kabul, is that if key figures such as Ahmed Wali Karzai and Jan Mohammed Khan can be killed in their own homes, how can ordinary Afghans be protected against insurgents?

Though a close ally of the president, Mr Khan - a former governor of Uruzgan province - was also a controversial figure.

Correspondents say that despite rising casualty numbers, security has improved in a number of areas of the country following a surge of extra American troops and tens of thousands of new Afghan police and soldiers.

However the quality of Afghan police and soldiers is patchy and correspondents warn of fears that they will be unable to withstand a renewed summer offensive from the Taliban.