Praise be to Allaah.
If the accident is major and serious, then it is OK to stop one’s prayer; if it is a minor incident that can be dealt with after completing the prayer, then it is not permissible. The scholars said: it is obligatory to stop one's prayer in order to save a negligent person from a fatal accident. For example, if you are praying and you see a snake approaching a man who is sitting down and has not noticed it. If it is heading for him, you have to stop your prayer and tell him so that he will not be bitten, possibly fatally. Or if you see a blind man walking in front of you whilst you are praying, and he is heading towards a well, and you are afraid that he may fall into it, then you have to stop your prayer and save him from that fatal accident, even if this means stopping your prayer. The same applies in cases of fire, etc.
But if the siren goes off for a minor matter which can be dealt with after completing the prayer and it will not be too late if you complete the prayer first, then it is not permissible to stop your prayer before you have completed it, because this accident can be dealt with after completing the prayer, without anyone coming to further harm. And Allaah knows best.