WASHINGTON - US Secretary of State John Kerry has strongly condemned Wednesday's "heinous" attack on a bus in Karachi, and offered American assistance in investigating the incident in which dozens of Ismaili community members were killed.

"The American people stand in solidarity with the people of Pakistan, and with the global Ismaili community on this tragic day," Kerry said in a statement issued by the State Department on Thursday.

"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 said.

"I extend my personal condolences to the families of the victims, and to my dear, esteemed friend His Highness 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 added, "We will support efforts to bring all those responsible to justice and stand ready to provide assistance to the investigation of this tragic attack.