U.N. Secretary-General Ban Ki-moon said Norwegian Foreign Minster Jonas Gahr Stoere, who was not hurt in the attack, was the target of the assault, which came as the Norwegian Embassy was holding a meeting at the Serena Hotel in the heavily protected Afghan capital, according to the Associated Press.
Two State Department officials told the news agency that at least one American was among the dead although other media reports varied on whether any foreigners were killed in the attack.
Ban said Stoere was the target but could not say why.
"They do not care whoever, whatever. This is really a serious crime against humanity," Ban told reporters at U.N. headquarters in New York City.
The Serena Hotel is Afghanistan's only five-star hotel and has been open since 2006, the New York Times reported. It is popular with diplomats and other Western workers. The attack was the deadliest assault on a hotel in Kabul since the fall of the Taliban in 2001.
The attackers were armed with suicide vests, grenades and AK-47 rifles and appeared to focus their siege on the hotel's gym and spa, according to media reports.
Suzanne Griffin, a Seattle resident who works with the aid agency Save the Children, told the AP she was in the gym's locker room when the attack started.
"Thank God I didn't get into the shower because then we heard gunfire, a lot of it. It was very close, close enough that plaster came off the ceiling," said Griffin. "We all just sat on the floor and got as far as we could from any glass and huddled on the floor. We turned our phones on silent."
The militants killed six people and wounded six, said Interior Ministry spokesman Zemeri Bashary. One of the attackers was shot to death and the Taliban spokesman said a second died in the suicide explosion.
Zabiullah Mujahid, the Taliban spokesman, told news agencies that four militants with suicide vests attacked the hotel -- one bomber who detonated his explosives and three militants who threw grenades and fired guns and then fled. The claim could not be independently verified.
In Washington, two State Department officials said that at least one American was among the dead. The identity of the victim was being withheld until family could be notified, the official said, speaking on condition of anonymity, according to the AP.
White House press secretary Dana Perino told reporters it unclear who was responsible for the hotel attack.
"It underscores the reason we have to stay on the offense against the extremists in places like Kabul but also in other places around the world," she said.
The attack comes as the U.S. considers adding some 3,000 U.S. Marines to aid in its efforts in Afghanistan and thwart an expected Taliban offensive in the spring.
Defense officials said Monday that Defense Secretary Robert Gates has recommended to the White House that additional troops be sent to Afghanistan to augment NATO forces, but no final decision has been made on the deployment.
"A recommendation has been forwarded for discussion. But at this point, no decision has been made. We're still waiting," Pentagon spokesman Army Lt. Col. Mark Wright told Reuters.