A suicide bomber on Tuesday killed former Afghan president and head of the country's High Peace Council Burhanuddin Rabbani, who was spearheading peace efforts with the Taliban.
Police said the bomber entered Mr. Rabbani's house late Tuesday for peace talks, and detonated a bomb hidden in his turban while greeting the former president.
In a written statement, NATO officials blamed two suicide bombers for the attack, saying they were feigning a desire to conduct reconciliation talks.
At least four other people were killed in the blast and one of President Hamid Karzai's key advisors, Masoom Stanekzai, was wounded.
Mr. Karzai met with U.S. President Barack Obama in New York Tuesday on the sidelines of the annual United Nations General Assembly meeting, but cut short his visit because of the attack. He was set to address the General Assembly on Wednesday.
Before their meeting, the two presidents condemned Mr. Rabbani's assassination. Mr. Obama said Burhanuddin Rabbani was a man who cared deeply about Afghanistan. The president said he will strengthen U.S. resolve to work with the Afghans for peace.
Mr. Karzai said the former Afghan president sacrificed his life for the sake of Afghanistan and peace.
U.S. Secretary of State Hillary Clinton extended her condolences to Mr. Rabbani's family and to all the people of Afghanistan. She said Washington will support the Afghan government "as they pursue the ones responsible for this cowardly attack and bring them to justice."
U.S. Defense Secretary Leon Panetta said these types of attacks and assassinations are a concern, but that overall, he believes NATO and the Afghan government are moving in the right direction against the insurgency
NATO Secretary-General Anders Fogh Rasmussen vowed that "those who offer only death and destruction" to the Afghan people will not prevail.
British Prime Minister David Cameron said Mr. Rabbani played a vital role in Afghanistan's peace process and that he was "absolutely appalled" by his murder.
Pakistani leaders also expressed "extreme anger and shock" at Mr. Rabbani's killing. In a statement, they accused "enemies of peace in Afghanistan" for his death.
Some information for this report was provided by AFP, AP and Reuters.