Security officials cordon off the area at the scene of an attack on a bus in Karachi, Pakistan, May 13, 2015. PHOTO: REUTERS
"We will support efforts to bring all those responsible to justice and stand ready to provide assistance to the investigation of this tragic attack," US Secretary of State John Kerry said.
Pistol-wielding gunmen had stormed the bus carrying members of the Ismaili community on Wednesday.
Read: Macabre violence: Ugly face of terror
The claims by the Islamic State (IS) militants, posted on Twitter, are likely to raise fears over the Middle East-based militants' growing influence after they announced in January the creation of a branch in what they called "Khorasan province", encompassing Afghanistan, Pakistan and parts of surrounding countries.
US Secretary of State John Kerry condemned what he called a "heinous attack," adding the "American people stand in solidarity with the people of Pakistan."
"Make no mistake: There is more strength by far in the respect and solidarity that we feel towards one another than there could ever be in any terrorist attack," he added in a statement.
Kerry offered his condolences to the victims and their families as well as "the Aga Khan, who has led the Ismaili community in investing in so many important development and education projects not only in Pakistan, but around the world."
Read: Aga Khan shocked, saddened by attack on Ismailis in Karachi
On Thursday, Kerry was taking part in a summit at Camp David with Gulf leaders called by President Barack Obama in part to coordinate efforts to combat IS, which has captured large areas of Iraq and Syria.
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