An American and an Italian held hostage by al Qaida, as well as two Americans working with the terror group, were inadvertently killed by US drone strikes earlier this year, the White House said.

President Barack Obama said he took "full responsibility" for the counter-terror missions and offered his "grief and condolences" to the families of the hostages.

Mr Obama defended the legality of the January drone strike that killed the hostages and said there had been no evidence that the two men were present at what the US had determined was an al Qaida compound.

"Based on the intelligence that we had obtained at the time, including hundreds of hours of surveillance, we believed that this was an al Qaida compound, that no civilians were present and that capturing these terrorists was not possible," Mr Obama said at the White House. "And we do believe that the operation did take out dangerous members of al Qaida."

Among those believed killed in the strike was Ahmed Farouq, who the White House said was an American who was an al Qaida leader.

US officials have also concluded that Adam Gadahn, an American who had served as a spokesman for the terror network, was killed in a separate operation in January.

The White House said neither Farouq nor Gadahn were intentionally targeted in the strikes and the US did not have information indicating their presence at the site of the operations.

A US government official said the drone strikes occurred on January 14 and January 19 in the border region between Afghanistan and Pakistan.

The president made no mention of Farouq and Gadahn. Instead, he focused his remarks on American Warren Weinstein, who had been held by al Qaida since 2011, and Italian Giovanni Lo Porto, who had been held since 2012.

Mr Obama expressed regret for the deaths of the two men and offered condolences to their families.

"I realize there are no words that can ever equal their loss," he said.

Mr Weinstein, a 73-year-old development worker was abducted in August 2011. His capture came four days before his seven-year stint with the US Agency for International Development was to end.

In a message released previously in English and Arabic and addressed to Mr Weinstein's family, al Qaida said it was "not interested in keeping" Mr Weinstein but wanted to exchange him for prisoners in US custody.

Lo Porto was an Italian aid worker who went missing in Pakistan in January 2012.