Pakistan's government has insisted an explosion at a hotel in Islamabad was not a terrorist attack but was probably caused by a short-circuit.
At least seven people were hurt in the hotel explosion
At least seven people were hurt, including a US diplomat, when the explosion shattered the ground floor of the Marriott hotel on Thursday night.
An interior ministry spokesman said there was no evidence of sabotage.
However, a risk unit that advises the US State Department said it was "an improvised explosive device".
The blast blew out nearly all the windows of the ground floor and could be heard a kilometre away.
The State Department said 11 US embassy staff were having an evening meal at the Marriott at the time.
The embassy has barred its staff from going to the hotel until further notice.
The State Department has not commented officially, but an official told AFP: "I've seen the pictures and it sure doesn't look like this was an electrical problem."
The department-run Overseas Security Advisory Council risk assessment unit said: "An improvised explosive device bombing occurred at the Marriott hotel in Islamabad."
At least two Italians and the prime minister's chief security officer were among the injured. One hotel employee was reportedly in a critical condition.
Pakistan newspaper, The News, named the security officer as Syed Pervez Zahoor and said he told journalists at hospital "it was 99% a bomb blast".
A hotel employee, Khurram Ahmed, told AFP: "It seems to be a bomb explosion. The bomb was apparently hidden in one of the flower pots near the entrance of the hotel. The blast was huge. It shook everyone in the hotel."
Information Minister Sheikh Rashid Ahmed said reports of a bomb were "not true".
"It was just an accident caused by short-circuiting and nothing else," he said.
Brigadier Javed Cheema of the interior ministry's national crisis management unit told AFP: "We haven't found any evidence that would suggest that it was an act of sabotage.
"The evidence indicates the blast was caused by short-circuiting. At the moment we rule out the possibility of any act of terrorism."
Brigadier Cheema said there could have been an air vacuum in the hotel.
"If your room is virtually airtight, even if one electricity bulb bursts, it produces a big sound," he said.
"These buildings are under high pressure so hence the bang was created."
The Marriott is one of the most popular hotels in Islamabad for foreign media and business people.