By Paul Tait
KABUL (Reuters) - General David Petraeus, Washington's new intelligence chief, handed over command of U.S. and NATO-led troops in Afghanistan Monday, a day after a tentative start was made to a gradual process of transferring security to Afghan forces.
Petraeus, credited with reversing a spiral toward civil war in Iraq, took over in Afghanistan a year ago after his predecessor, General Stanley McChrystal, was sacked by President Barack Obama for comments made in a magazine story.
He is leaving the military to take over as director of the Central Intelligence Agency as part of a wider shake-up of senior U.S. security officials and takes over from Leon Panetta, the new U.S. defense secretary.
Petraeus, who hands over to U.S. Marine Corps General John Allen, oversaw a "surge" of 30,000 extra U.S. forces which helped stop the momentum of a growing insurgency, especially in the Taliban heartland in the south. He led a similar escalation of forces that helped turn around the Iraq conflict in 2007-08.
However, despite gains in violent southern provinces during Petraeus' year in charge, the Taliban-led insurgency is still far from quelled.
Violence across Afghanistan in 2010 hit its worst levels since the Taliban were ousted by U.S.-led Afghan forces in 2001, with civilian and military casualties hitting record levels, and this year has followed a similar trend.
In an especially gruesome incident, two Afghans were beheaded in Afghanistan's west Monday, villagers and police said. They were kidnapped along with 33 others last week for apparently supporting the Afghan government.
Their beheaded bodies were sent back to their families.
In the south, the police chief of a district in Kandahar -- the birthplace of the Taliban -- and three other officers were killed by a roadside bomb, the provincial government said.
"We should be clear-eyed about the challenges that lie ahead," Petraeus, who is expected to take over at the CIA in September, said at a ceremony for the change of command of the NATO-led International Security Assistance Force (ISAF).
Underlining those challenges, ISAF said Monday three of its troops had been killed by a homemade bomb in Afghanistan's east, where some of the toughest fighting has taken place over the past year and where a fractured insurgency still rages.
Some analysts have questioned the success of Petraeus' much-vaunted counter-insurgency strategy in the face of rising violence. But Allen vowed to press ahead.
"It is my intention to maintain the momentum of this great campaign on which we have embarked," Allen said. "There will be tough days ahead."
The United States and other ISAF nations are rapidly trying to train tens of thousands of extra Afghan police and soldiers so that they can eventually take over from the roughly 150,000 foreign troops in Afghanistan.
Sunday, ISAF handed security control over to Afghan forces in central Bamiyan province, marking the start of a gradual transition process that will end with all foreign combat troops leaving Afghanistan by the end of 2014.
Bamiyan, one of the safest of Afghanistan's 34 provinces and long an anti-Taliban redoubt, was the first of seven areas to be handed over during the first phase of transition.
Lashkar Gah, the capital of volatile Helmand province and the most contentious of the first seven areas, will be handed over officially Wednesday.
U.S., British and Afghan troops swept through the sparsely populated desert outside Lashkar Gah at the weekend to root out Taliban fighters who might try to disrupt the transfer.
"This is very important for our security. It's also important for the transition of Lashkar Gah. We will make a screen for the Taliban not to cross or bother our security build-up," Afghan Army Major Mohaib told Reuters.
The first phase of transfers -- mostly in areas considered relatively safe -- will be a critical test of the readiness of Afghan forces and of NATO plans to allow the United States and its allies to take their troops off the front line of an increasingly unpopular war.
But the frequency and scale of recent attacks have been a worrying sign as the transition process begins.
Late Sunday, gunmen killed an adviser to Afghan President Hamid Karzai and another Afghan lawmaker, days after Ahmed Wali Karzai, the president's half-brother and one of the most powerful men in Kandahar, was killed.
Barely a month ago, Taliban gunmen and suicide bombers attacked a leading hotel in the capital in a raid in which at least 12 people were killed and which only ended when ISAF helicopters were called in to kill the remaining attackers.
Such killings -- many claimed by the Taliban -- have sent chilling warnings to political leaders about the reach of the Taliban, who have shown an ability to adapt their tactics even as their strength has been diminished.
(Additional reporting by Sharafuddin Sharafyar in Herat, Adrian Croft in Helmand, and Mirwais Harooni and Emma Graham-Harrison in Kabul; Editing by Yoko Nishikawa)