Fighting between the two sides escalated sharply on Thursday with a first-ever militant attack on the Tel Aviv area, menacing Israel's heartland. No casualties were reported, but three people died in the country's rocket-scarred south when a projectile slammed into an apartment building.

The death toll in the densely populated Palestinian territory climbed to 19, including five children according to Palestinian health officials, as waves of Israeli fighter planes and drones sent missiles hurtling down on suspected weapons stores and rocket-launching sites.

Early Friday, 85 missiles exploded within 45 minutes in Gaza City, sending black pillars of smoke towering above the coastal strip's largest city. The military said it was targeting underground rocket-launching sites.

One missile hit the Interior Ministry, a symbol of Hamas power.

The fighting has already widened the instability gripping a region in the throes of war and regime upheavals. Most immediately, it is straining already frayed relations with Egypt, which plans to send its prime minister to Gaza later Friday in a show of solidarity with its militant Hamas rulers.

Israel and Hamas had largely observed an informal truce since Israel's devastating incursion into Gaza four years ago, but rocket fire and Israeli airstrikes on militant operations didn't halt entirely. The latest flare-up exploded into major violence Wednesday when Israel assassinated Hamas' military chief, following up with a punishing air assault meant to cripple the militants' ability to terrorize Israel with rockets.

The Israeli military reported early Friday that its aircraft had struck more than 350 targets since the beginning of its operation against Hamas' rocket operations.

On Thursday, Israeli warplanes struck dozens of Hamas-linked targets, sending loud booms echoing across the narrow Mediterranean coastal strip at regular intervals, followed by gray columns of smoke. After nightfall, several explosions shook Gaza City several minutes apart, a sign the strikes were not letting up. The military said the targets were about 70 underground rocket-launching sites.

The onslaught has not deterred the militants from striking back with more than 400 rockets aimed at southern Israel. For the first time, they also unleashed the most powerful weapons in their arsenal _ Iranian-made Fajr-5 rockets capable of reaching Tel Aviv.

The two rockets that struck closest to Tel Aviv appear to have landed in the Mediterranean Sea, defense officials said, and another hit an open area on Tel Aviv's southern outskirts.

No injuries were reported, but the rocket fire _ the first in the area from Gaza _ sowed panic in Tel Aviv and made the prospect of a ground incursion more likely. The government later approved the mobilization of up to 30,000 reservists for a possible invasion.

Israeli Prime Minister Benjamin Netanyahu said the army was hitting Hamas hard with what he called surgical strikes, and warned of a ``significant widening'' of the Gaza operation. Israel will ``continue to take whatever action is necessary to defend our people,'' said Netanyahu, who is up for re-election in January.

At least 12 trucks were seen transporting tanks and armored personnel carriers toward Gaza late Thursday, and buses carrying soldiers headed toward the border area. Israeli TV stations said a Gaza operation was expected on Friday, though military officials said no decision had been made.

``We will continue the attacks and we will increase the attacks, and I believe we will obtain our objectives,'' said Lt. Gen. Benny Gantz, Israel's military chief.

An Israeli ground offensive could be costly to both sides. In the last Gaza war, Israel devastated large areas of the territory, setting back Hamas' fighting capabilities but also paying the price of increasing diplomatic isolation because of a civilian death toll numbering in the hundreds.

The current round of fighting is reminiscent of the first days of that three-week offensive against Hamas. Israel also caught Hamas off-guard then with a barrage of missile strikes and threatened to follow up with a ground offensive.

However, much has also changed since then.

Israel has improved its missile defense systems, but is facing a more heavily armed Hamas. Israel estimates militants possess 12,000 rockets, including more sophisticated weapons from Iran and from Libyan stockpiles plundered after the fall of Moammar Gadhafi's regime there last year.

Netanyahu, who has clashed even with his allies over the deadlock in Mideast peace efforts, appears to have less diplomatic leeway than his predecessor, Ehud Olmert, making a lengthy military offensive harder to sustain.

What's more, regional alignments have changed dramatically since the last Gaza war. Hamas has emerged from its political isolation as its parent movement, the region-wide Muslim Brotherhood, rose to power in several countries in the wake of last year's Arab uprisings, particularly in Egypt.

Egypt recalled its ambassador to protest the Israeli offensive and has ordered his prime minister to lead a senior delegation to Gaza on Friday in a show of support for Hamas.