The Israeli campaign has been limited to airstrikes so far. But military officials say they are considering expanding it to a ground campaign.

Lt. Col. Avital Leibovich, a military spokeswoman, said the military had called 16,000 reservists to duty on Friday as it geared up for a possible ground offensive.

She said the army had authority to draft an additional 14,000 soldiers. She would not say where the troops were deployed.

As air-raid sirens went off in Jerusalem, witnesses said they saw a stream of smoke in Mevasseret Zion, a Jerusalem suburb.

Israeli police spokesman Micky Rosenfeld said the rocket landed in an open area near Gush Ezion, a collection of Jewish settlements in the West Bank southeast of the city.

An attack on Israel's self-declared capital marks a major escalation by Gaza militants, both for its symbolism and its distance from the Palestinian territory. Located roughly 50 miles away from the Gaza border, Jerusalem had been thought to be beyond the range of Gaza rocket squads.

Abu Obeida, spokesman for the Hamas militant wing, said the group had fired a long-range rocket at Jerusalem.

"We are sending a short and simple message: There is no security for any Zionist on any single inch of Palestine and we plan more surprises," he said. Hamas officials said the rocket was a homemade "M-75" rocket, a weapon that has never been fired before.