General John Allen, has taken over the command of US-led international troops in Afghanistan from General David Petraeus, hours after a senior aide to Afghan President Hamid Karzai was killed in another high-profile assassination.
Allen was promoted to a four-star general shortly before the handover ceremony in Kabul, the Afghan capital, on Monday, to replace his former superior Petraeus.
Petraeus is ending his year of service at the post to head up the Central Intelligence Agency.
At the ceremony, Allen said US forces' drawdown, which started earlier this month, did not mean international forces were easing up in their campaign to defeat the Taliban.
"It is my intention to maintain the momentum of the campaign,'' Allen said. "There will be tough days ahead. I have no illusions about the challenges ahead.''
Al Jazeera's Bernard Smith, reporting from Kabul, said questions remain about the ability of the Afghan forces as security transition becomes the focus.
"This week, we are seeing the transition take place from foreign forces to Afghan forces. The concern here all the time is whether the Afghan police and national army are ready," he said.
"Many believe they are not."
Assaults and assassinations
U.S. officials have trumpeted success in reclaiming Taliban strongholds in southern Afghanistan and training Afghan security forces, as signs that they are finally making progress toward peace in Afghanistan.
But violent attacks have continued, including a number of high-profile assaults and assassinations in recent weeks.
Sunday night's killing of the president's senior adviser Jan Mohammad Khan, a former governor of the southern Uruzgan province, follows the killing last week of Ahmed Wali Karzai, probably the most powerful man in southern Afghanistan and younger half-brother of the Afghan president.
An Afghan member of parliament, Mohammad Hashim Watanwal, also died in a raid on Khan's home by two gunmen wearing suicide vests.
The Taliban claimed responsibility for the deaths of both Khan and Wali Karzai.
On Monday morning, a bomb killed three members of the international security force in the east, NATO said in a statement. It did not provide nationalities or further details.
Like his predecessor, Allen had made his name in Iraq by striking tribal alliances considered integral in reversing al Qaeda's momentum after years of appalling violence.
Petraeus had also been celebrated for his work in Iraq when he took charge in Afghanistan last year, after US President Barack Obama sacked his predecessor Stanley McChrystal over scathing remarks made to Rolling Stone magazine about the White House administration.
Petraeus oversaw a surge of over 30,000 troops into Afghanistan in a bid to reverse the Taliban insurgency but violence remains at record highs, despite his claims of some progress.
UN statistics released last week show that 1,462 civilians died in the first six months of 2011, an increase of 15 per cent, and putting this year on track to be the deadliest in a decade.