Time : ` ` The situation is tense , '' a Yemeni security official said . It is not clear what set off the explosion . There are no immediate reports of casualties . Trains to Sanaa are being diverted to Aden .
NEW : `` The situation is tense , '' a Yemeni government official says . It is not clear what set off the explosion . There are no immediate reports of injuries . Flights to Sanaa are being rerouted to Aden .