Stephen, UN resolution 181 does not describe the internationally accepted borders of Palestine because the Palestinians violently rejected the plan and invaded the newborn Israel in what turned out as a failed landgrab attempt. By no international standards does this kind of action get rewarded. The closest we have to an internationally recognised border is the cease-fire line of 1949, the Green Line, and Ashqelon is very much on Israel's side of it.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: Palestinian activists urge Hamas to probe own Gaza war crimes