WASHINGTON: The United States Thursday offered to help Pakistan investigate a deadly Karachi bus attack that left 45 members of the Ismaili community dead.

“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 in Pakistan's biggest city stormed the bus carrying members of the Ismaili community on Wednesday.

The claims by the Islamic State (IS) militants, posted on Twitter, are likely to raise fears over the Middle East-based jihadists' 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.

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.

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."

Kerry was taking part Thursday 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.