GAZA CITY, Gaza Strip – Palestinian militants took aim at Jerusalem for the first time Friday, launching a rocket attack on the holy city in a major escalation of hostilities as Israel pressed forward with a relentless campaign of airstrikes in the Gaza Strip.

Israel called up thousands of reservists and massed troops along the border with Gaza, signaling a ground invasion of the densely populated seaside strip could be imminent. The attack on Jerusalem, along with an earlier strike on the metropolis of Tel Aviv, raised the likelihood that Israel would soon move in.

Israel triggered the current fighting by assassinating the military chief of the ruling Hamas militant group on Wednesday, then conducting dozens of airstrikes on weapons-storage sites used by rocket squads. The sudden attack came in response to days of heavy rocket fire from Gaza.