Afghan security forces and coalition troops are responding to the assault at the site at Qargha Lake on the outskirts of the Afghan capital, a spokesman for NATO's International Security Assistance Force (ISAF) told AFP.
The target appeared to be Afghan police and civilians, ISAF said, adding there were reports of police and civilian casualties.
The assault came just hours after President Hamid Karzai warned that attacks against local police and soldiers were increasing as they prepare to take over security when NATO combat troops leave in 2014.
Mohammad Zahir, the head of Kabul police criminal investigation department, said the attack began around 11:30 pm (1900 GMT) on Thursday, when suicide attackers armed with rockets and Kalashnikov rifles stormed a gathering at the Spozhmai Hotel.
At least one of the attackers detonated his explosive suicide vest, Zahir said.
Kabul deputy police chief Daoud Amin said that according to hotel guards, there were three attackers inside the hotel, as well as numerous civilians.
"Currently, police forces have besieged the hotel where people gathered for a picnic or wedding party," he told AFP.
"We will start the operation against them soon and we will be very careful to make sure that civilians are not harmed."
Afghan troops with around a dozen armoured vehicles were standing by around 500 metres from the hotel, along with police and ambulances, according to an AFP photographer at the scene.
Taliban violence has surged in recent days with a series of attacks on Afghan-NATO military posts.
This week's biggest attack came on Wednesday when a suicide bomber on a motorbike rammed a joint Afghan-NATO patrol in the eastern city of Khost, close to the Pakistan border, killing 21 people including three US soldiers.
The hotel attack will heighten fears about security as NATO prepares to hand responsibility to Afghan forces and recall the vast majority of its 130,000 combat troops.
Karzai on Thursday admitted his government and its Western allies had failed to bring peace to Afghanistan, which has suffered almost continuous conflict for the past three decades, saying "our land has not been secured, our homes, our people are not safe".
In April militants launched coordinated attacks on government offices, embassies and foreign bases in Kabul in the biggest assault on the Afghan capital in 10 years of war.
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