The rocket attacks were a challenge to Israel's Gaza offensive and came just hours after Egypt's prime minister, denouncing what he described as Israeli aggression, visited the enclave and said Cairo was prepared to mediate.

Israel's armed forces announced that a highway leading to the Gaza Strip and two roads bordering the enclave would be off-limits to civilian traffic until further notice.

Tanks and self-propelled guns were seen near the border area and the military said it had already called 16,000 reservists to active duty.

Prime Minister Benjamin Netanyahu convened senior cabinet ministers in Tel Aviv after the rockets struck to decide on widening the Gaza campaign.

Political sources said ministers were asked to approve the mobilisation of up to 75,000 reservists, in what could be preparation for a possible ground operation.

No decision was immediately announced and some commentators speculated in the Israeli media the move could be psychological warfare against Gaza's Hamas rulers. A quota of 30,000 reservists had been set earlier.

Israel began bombing Gaza on Wednesday with an attack that killed the Hamas military chief. It says its campaign is in response to Hamas missiles fired on its territory. Hamas stepped up rocket attacks in response.

Israeli police said a rocket fired from Gaza landed in the Jerusalem area, outside the city.

It was the first Palestinian rocket since 1970 to reach the vicinity of the holy city, which Israel claims as its capital, and was likely to spur an escalation in its three-day old air war against militants in Gaza.

Rockets nearly hit Tel Aviv on Thursday for the first time since Saddam Hussein's Iraq fired them during the 1991 Gulf War. An air raid siren rang out on Friday when the commercial centre was targeted again. Motorists crouched next to cars, many with their hands protecting their heads, while pedestrians scurried for cover in building stairwells.

The Jerusalem and Tel Aviv strikes have so far caused no casualties or damage, but could be political poison for Netanyahu, a conservative favoured to win re-election in January on the strength of his ability to guarantee security.

"The Israel Defence Forces will continue to hit Hamas hard and are prepared to broaden the action inside Gaza," Netanyahu said before the rocket attacks on the two cities.

Asked about Israel massing forces for a possible Gaza invasion, Hamas spokesman Sami Abu Zuhri said: "The Israelis should be aware of the grave results of such a raid and they should bring their body bags."

Officials in Gaza said 28 Palestinians had been killed in the enclave since Israel began the air offensive with the declared aim of stemming surges of rocket strikes that have disrupted life in southern Israeli towns.

The Palestinian dead include 12 militants and 16 civilians, among them eight children and a pregnant woman. Three Israelis were killed by a rocket on Thursday. A Hamas source said the Israeli air force launched an attack on the house of Hamas's commander for southern Gaza which resulted in the death of two civilians, one a child.