BEIRUT (Reuters) - The judge leading the investigation into last summer's Beirut port blast issued a subpoena for caretaker Prime Minister Hassan Diab on Thursday after he failed to show up for questioning, Lebanon's state news agency reported.

       Judge Tarek Bitar, who is leading the inquiry into the explosion, issued requests in July to question Diab and other top officials, including Major General Abbas Ibrahim, head of the powerful General Security agency, and several former ministers.

       The blast, caused by large quantities of ammonium nitrate stored unsafely for years, killed hundreds, injured thousands and destroyed large swathes of the capital.

       More than a year on, no senior official has been held accountable, angering many Lebanese.

       The inquiry into the explosion repeatedly stalled with the first lead judge removed in February after a court granted the request of two of the former ministers he had charged with negligence for the disaster.

       Requests by Bitar to lift the immunity of several members of parliament and to question top officials have also stalled.

       Diab's session was postponed to Sept. 20, the news agency said.

       A judicial source told Reuters that should Diab fail to attend the September session, the judge would have the right to issue an arrest warrant.

       Diab, who has said he is innocent of any wrongdoing connected to the blast, was not immediately available for comment.

       (Reporting by Maha El Dahan and Laila Bassam; Editing by Nick Macfie)