JERUSALEM -- Israel massed tanks and troops along Gaza's northern border early today, firing artillery and unleashing more airstrikes in a show of force after the prime minister ordered his army to "do all it can" to free an abducted soldier.
At daybreak, a small force of Israeli tanks entered northern Gaza, but the military said it was a "limited" mission to find explosives and tunnels near the border fence.
Prime Minister Ehud Olmert's warning signaled the government was losing patience with diplomatic efforts to end the week-old crisis over a captive soldier, Cpl. Gilad Shalit, 19, and was preparing for a possible escalation of its military offensive. Israeli artillery and naval vessels shelled the coastal strip for the fifth straight day.
Israel did allow food shipments yesterday through the main cargo crossing and restored fuel supplies to the Palestinians amid warnings of a humanitarian crisis in the Gaza Strip.
The Israeli moves to allow in supplies came after United Nations and Red Cross officials expressed concern about deteriorating conditions in Gaza in the wake of Israeli airstrikes last week that knocked out power.
Secretary of State Condoleezza Rice voiced similar concerns in a phone call yesterday to Olmert, who replied that there was no humanitarian crisis in Gaza and that Israel had allowed supplies into the area, his office said.
More than 50 trucks carrying milk, meat, wheat, flour, cooking oil, fruit and other staples moved into Gaza through the Karni Crossing, and diesel fuel, gasoline and natural gas were transferred through pipelines, army officials said. Israel shut the crossing and stopped supplying fuel after the Palestinian attack last week.
The destruction of Gaza's only power plant in an Israeli airstrike Wednesday knocked out 43 percent of the electricity in the territory, leading to increased demand for diesel fuel to run generators at hospitals and water pumping stations.
Power routinely supplied by Israel to parts of the Gaza Strip is now being distributed for periods of six hours, residents say. Palestinians have accused Israel of worsening conditions for Gazans in an effort to turn them against the Hamas-led government and create public pressure for the soldier's release.
Olmert insists that Israel "is not interested in harming the Palestinian population."
But Interior Minister Roni Bar-On, a close ally, said civilian hardship is part of Olmert's plan.
"The prime minister explicitly directed the army that no one should sleep at night in Gaza - explicitly that there should be discomfort there," Bar-On told Israel Radio. "The equation is very clear: If people won't live at peace here, they won't live at peace in Gaza."
Early today, Israeli aircraft struck a building in Gaza City that the army said was used as an operations center by the Aqsa Martyrs Brigades, a militant group linked to the Fatah party of Palestinian President Mahmoud Abbas.
The attack came nearly 24 hours after Israeli aircraft rocketed the empty office of Palestinian Prime Minister Ismail Haniyeh of Hamas. Abbas joined Haniyeh in inspecting the damage and condemned what he called "the destruction of the civilian institutions of the Palestinian people."
A spokesman for the armed wing of Hamas threatened to attack targets in Israel similar to those struck by Israel in Gaza.
The continuing military action came as Egyptian mediators tried to broker a deal for the release of the soldier, with no apparent progress. The Egyptians were reportedly working on a proposal that the soldier would be freed in exchange for an Egyptian commitment that Israel would release prisoners at an unspecified later date.
The armed wing of Hamas and two allied groups holding the soldier, the Popular Resistance Committees and the Army of Islam, have demanded the release of 1,000 Palestinian and other Arab prisoners held in Israeli jails, the freeing of 500 imprisoned women and minors, and a halt to the Israeli offensive.
Israel, which has exchanged Arab prisoners for captured servicemen in the past, rejected any prisoner swap.
"These are not easy days for the state of Israel, but we have no intention of surrendering to blackmail," Olmert said yesterday at the start of the weekly Cabinet meeting. "Everyone knows that surrendering to terrorism today means inviting the next act of terrorism."
Abbas' office said Saturday that negotiations were being hampered by "the absence of an address on the Hamas side," and that Khaled Mashaal, the leader of Hamas, who is based in Syria, was passing responsibility to Hamas in Gaza while the armed wing was saying decisions rested with him.
Last week, Israeli tanks and troops entered the southern Gaza Strip, where the captive soldier is believed to be held, taking over an unused airport near Rafah. Troops killed three militants there last night, two of whom were wearing explosive belts, the army said.
Israel pressed Hamas by arresting dozens of its political leaders in the West Bank last week, including eight Palestinian Cabinet ministers and more than 20 lawmakers.
Joel Greenberg writes for the Chicago Tribune. The Associated Press contributed to this article.