The United Nations Relief and Works Agency apologised to the US government last night after it was criticised for passing to a US senator a letter from a Hamas representative intended for President Barack Obama.

Karen Koning AbuZayd, the agency head, said she "deeply regrets any awkwardness" the transmission of the letter may have caused for the government, which has refused any direct contact with the Palestinian Islamist faction that rules Gaza.

The agency denies claims by Israeli officials that it was embroiling itself in politics by passing the letter to John Kerry, the chairman of the Senate Foreign Relations committee, when he visited Gaza last week. Mr Kerry left the letter with the US consulate in Jerusalem. He said he had not read it because it was sandwiched among UN promotional papers he had received.

Christopher Gunness, the agency's chief spokesman, said guards at the agency's front gates sometimes received envelopes addressed to people outside Gaza. "Provided there is nothing to suggest anything untoward, our practice is to forward these letters, in good faith, as best we can."

The letter to Mr Obama, with a Palestinian Authority return address, had been handled in this way. Mr Gunness said the incident did not detract from the UN agency's neutrality.