Israel's two-day assault on Gaza has largely been from the air, but officials have said a ground incursion could be necessary to stem rocket fire into southern Israel.
Israel's army would be heavily dependent on reservists to fight any prolonged war. The military has received a green light to call in up to 30,000 reserve troops.
The Palestinian Authority has renewed its call for the U.N.Security Council to take action to stop Israel's attacks against the Gaza Strip, a day after the 15-nation body held an inconclusive emergency meeting on the escalating conflict.
"The mobilization of Israeli occupying forces on the ground, including the amassing of tanks, armored vehicles and buses near the Gaza border are cause for serious concern and demand the attention of the international community," the Palestinian U.N. Observer, Riyad Mansour, wrote to the U.N. Security Council.
"We thus reiterate our urgent appeal to the Security Council... to uphold its (U.N.) Charter duties and to act now to protect the Palestinian civilian population underIsrael's occupation in accordance with international humanitarian law," he wrote to Indian Ambassador Hardeep Singh Puri, president of the Security Council this month.
Israel launched a major offensive against Palestinian Hamas militants in Gaza on Wednesday (local time), killing Hamas' military commander in an air strike and threatening an invasion of the enclave that the Islamist group said would "open the gates of hell."
Israeli Ambassador Ron Prosor called for international condemnation of the "indiscriminate rocket fire against Israeli citizens - children, women," referring to recent escalating Palestinian rocket attacks from Gaza.
The U.N. Security Council held an emergency meeting to discuss Israel's strikes against Gaza but took no action, as Israel threatened a wider offensive in the Palestinian enclave to stem rocket salvoes by Hamas militants.
Puri told reporters after Wednesday's meeting that the council would convene again on the Gaza crisis if necessary.
Mansour said in his letter to Puri that Israel's attacks on Gaza were "a grave breach of all norms of international law."
Hamas, not the Palestinian Authority, controls Gaza.
U.N diplomats said earlier on Thursday that U.N. Secretary-General Ban Ki-moon would visit Israel and Egypt next week to help mediate an end to the conflict.
The Security Council's failure to take any action on Wednesday was not a surprise. It is generally deadlocked on the Israeli-Palestinian conflict, which U.N. diplomats say is due to the U.S. determination to protect Israel.
The fighting escalated on Thursday. Two rockets from Gaza crashed near Tel Aviv in the first such attack on Israel's commercial capital in 20 years. One fell into the Mediterranean Sea and the other in an uninhabited part of one of the Tel Aviv suburbs south of the city.
Two days of Israeli air strikes have killed 19 Palestinians - seven militants and 12 civilians - including six children and a pregnant woman. A Hamas rocket killed three Israelis in the town of Kiryat Malachi on Thursday morning.