JERUSALEM — Israeli forces Saturday made their deepest and deadliest incursion into the Gaza Strip in years, attacking Palestinian rocket-launching squads with tanks and warplanes but also inflicting a heavy loss of civilian life in the densely populated enclave.

Fifty-four Palestinians, 26 of them civilians, and two Israeli soldiers were reported killed as fighting raged into the night around militant strongholds in two urban centers of northern Gaza.

Trapped inside by the fighting, some Palestinians died when Israeli fire tore through their homes, witnesses said. But at least one civilian, a 13-month-old girl, was killed at home by a Palestinian rocket that fell short of its target and sprayed her with shrapnel, according to neighbors.

Palestinian hospital officials called it the bloodiest day of fighting in Gaza since 2000 and put the toll at 84 killed since a spike in the fighting Wednesday. They listed 150 Palestinians wounded Saturday.

The spiraling violence in Gaza, a seaside territory of 1.5 million people ruled by the Islamic movement Hamas, prompted a threat by the rival, secular-led Palestinian Authority based in the West Bank to suspend U.S.-backed peace talks with Israel.

Palestinian Authority President Mahmoud Abbas condemned the Israeli assault as "state terrorism" and called for an urgent session of the United Nations Security Council to press Israel to end it. Militant leaders and commentators across the Arab world referred to the bloodshed as a "holocaust."

Israeli government spokesman David Baker said Hamas and other militant groups bore responsibility for the civilian deaths. "They hide behind their own civilians, using them as human shields, while actively targeting Israeli population centers," he said.

Israel is "compelled to take these defensive measures" to protect 200,000 Israelis living within Palestinian rocket range, he added.

Many unarmed Gazans said they felt like intentional targets of the Israeli raids.

Israeli leaders have been debating for weeks whether to launch a lengthy, large-scale ground operation to try to halt the near-daily rain of homemade Kassam rockets from Gaza. Saturday's incursion appeared to be a short-term action with the more limited aim of pushing the rocket squads back from the border.

Still, it was the deepest and one of the largest Israeli incursions in Gaza since the Jewish state unilaterally withdrew its military bases and settlements from the territory in 2005.

Hundreds of Israeli armored and infantry soldiers crossed into northern Gaza before dawn, advanced several miles on militant strongholds in Beit Hanoun and Jabaliya, and dug in for what was expected to be several days of fighting.

After dark, Israeli warplanes killed seven members of the Hamas-led police force in southern Gaza, two in a car in Khan Yunis and five in a mosque being used at night as a police barracks in Rafah.

Three missiles fired from the air early today destroyed the Gaza City building housing Hamas Prime Minister Ismail Haniyeh's offices, which were vacant at the time.

The militants nonetheless kept up a steady barrage of rocket and mortar attacks throughout the day, aiming both at the soldiers and communities in Israel.

Israel reported seven soldiers wounded in addition to the two dead. They were the first army casualties since Israel intensified an aerial campaign Wednesday and Hamas replied with a rocket blitz that killed a middle-age student on a college campus in southern Israel.

Kassam rockets are wildly inaccurate, but militants have fired thousands of them from Gaza over the past seven years, killing 13 people and terrorizing Israeli communities a few miles away.

Since Thursday, the militants have raised the threat level by firing more than a dozen longer-range, Soviet-designed Grad missiles into the slightly more distant city of Ashkelon, 11 miles from Gaza. Israel says the rockets probably were made in Iran and smuggled into Gaza from Egypt.

Israeli Prime Minister Ehud Olmert has come under growing public pressure to act. But he is reluctant to order a full-scale ground operation in Gaza, in part because stopping the rocket fire would require re-occupying most of the strip and risking a prolonged period of bloodshed.

Also, with U.S. Secretary of State Condoleezza Rice scheduled to visit Israel this week, Olmert is unlikely to order a major escalation.

Rice is expected to arrive in Israel on Tuesday with an Egyptian proposal for a cease-fire in Gaza, an Israeli-Palestinian prisoner exchange and a reopening of the Egypt-Gaza border under European Union monitoring.

Israeli officials are skeptical of a cease-fire, which they fear would allow Hamas to continue stockpiling weapons through tunnels under the Egyptian border.

Rice will have an additional challenge: rescuing the peace talks begun in December between Israel and the West Bank-based Palestinian Authority. They are aimed at achieving agreement on the birth of an independent Palestinian state by the end of President Bush's term next January.