Palestinian fighters and the Israeli military have largely held their fire since a June 19 truce.
But the lull ended on Tuesday after Israeli troops launched an incursion into Gaza to destroy what they said was a tunnel dug near the border aimed at kidnapping Israeli soldiers.
Seven fighters were killed during the operation and dozens of rockets were fired into Israel in response.
A Palestinian fighter was killed on Wednesday in an Israeli air raid after two rockets were fired into Israel.
Israel has now closed its crossings into the impoverished territory, keeping much-needed goods from reaching the coastal strip of 1.4 million people.
Although the violence is the worst since the truce took effect, neither side seems to have much to gain from a renewal of hostilities, and both Israeli and Hamas officials have said they want to restore calm.