و عليكم السلام
1- I think (The city is on fire ) it give the meaning of المدينة يشتعل أجوائها like the city has problems that make it not rest, and sometimes fire = ضرب النار shoot with gun
The city is in flames>> I think it means in this case المدينة تحترق
and I think the whole sentence will indicate the meaning for you in both cases.
2-The fish is IN THE fire السمكة في النار as a cook way.