The United States strongly condemned the murder of Afghan President Hamid Karzai's younger brother - an influential ally in the US effort to quell the Taliban insurgency.
Secretary of State Hillary Clinton said she telephoned the Afghan president to offer condolences over Ahmed Wali Karzai, a controversial figure whom US officials had counted on to stand against the Taliban in Kandahar, the group's spiritual home.
"We join President Karzai in his prayer for peace and stability in Afghanistan and remain committed to supporting the government and people of Afghanistan in their struggle for peace."
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The US administration "condemns in the strongest possible terms the murder of President Karzai's half-brother in Kandahar," White House spokesman Jay Carney told reporters.
The younger Karzai was gunned down earlier Tuesday in his own home, and the Taliban quickly claimed responsibility for the killing. A provincial police chief said he was shot dead by a long-serving senior bodyguard.
US officials did not know who was responsible, but "we will certainly work with the Afghan authorities on that," Carney said.
Ahmed Wali Karzai was the government's key powerbroker in the south, and his murder deprives NATO of a vital if controversial ally.
He had been dogged by allegations of corruption and drugs links and was reported to have been on the CIA payroll.
But despite his unsavory reputation he played a crucial role for Americans in Kandahar, perhaps the toughest battlefield in 10 years of war and the focus of a troop surge last year.
For seven years he headed the provincial council in Kandahar, where he was widely considered to control all commercial and political dealings.
Leading members of the US Senate expressed doubts Tuesday about the scope of the assassination's impact.
"It's too early to make that kind of determination," Senate Foreign Relations Committee Chairman John Kerry told AFP, underlining that "on some things he was helpful, on some things he was not."
"I'm sorry for the Karzai family, for their loss. I know that President Karzai will take this personally," said Kerry, who added: "I think that it's hard to measure the impact."
Senator John McCain, the top Republican on the Senate Armed Services Committee and a critic of President Barack Obama's planned troop withdrawal from Afghanistan, said the killing was "an indication that we've got a lot of work to do in Afghanistan.
"And I don't think it has any particularly significant impact except that we've still got a pretty big challenge ahead of us," he added.
The killing raises disturbing questions about possible infiltration among those closest to the Karzai family and is also a severe blow to NATO and the Afghan leadership in Kandahar, the heartland of the Taliban insurgency.
"He was helpful in some ways, hurtful in others," said Senator Lindsey Graham, the top Republican on a key committee that shapes foreign aid.
"I don't really know" the impact on US policy, said Graham, who expressed hope that the vacuum left by the assassination would be filled by leaders whose message is "I can lead without any semblance of corruption."
The State Department stressed Tuesday there was no causal link between the killing and the troop reduction Obama announced last month.
Obama said 10,000 US troops would leave Afghanistan this year and all 33,000 forces sent as part of a surge ordered in late 2009 would be home by next summer.
