WASHINGTON (REUTERS) - A US citizen was among those killed in a militant attack in Bangladesh, the US government said on Saturday.

"We can confirm that a US citizen was also among those senselessly murdered in this attack," the State Department said, in a statement that gave no other details about the US citizen.

The White House condemned that attack in the Bangladeshi capital Dhaka, which it said resulted in as many as 20 deaths, including that of the American.

"We remain in contact with Bangladeshi authorities and have offered any assistance necessary," it said.

"This is a despicable act of terrorism, and the United States stands with Bangladesh and the international community in our resolve to confront terrorism wherever it occurs," it said.

At least nine Italians and seven Japanese were among the dead inside an upmarket restaurant in Bangladesh's capital, before security forces stormed the building and ended a 12-hour standoff on Saturday.

Islamic State in Iraq and Syria (ISIS) said it was responsible for one of the most brazen attacks in the South Asian nation's history, but that claim has yet to be confirmed.

It marked a major escalation in a campaign by militants over the past 18 months that had targeted mostly individuals advocating a secular or liberal lifestyle in majority-Muslim Bangladesh with 160 million people.