The call-up came after Washington announced it had agreed to restock Israel's dwindling supplies of ammunition despite its sharp condemnation of an attack on a United Nations school in Gaza blamed on Israel's army.
"The army has issued 16,000 additional mobilisation orders to allow troops on the ground to rest, which takes the total number of reservists to 86,000," an army spokeswoman said.
Israel's security cabinet, which met for five hours Wednesday, unanimously decided to pursue attacks against Hamas "terrorist targets" and other operations to destroy a network of tunnels used by the Islamist movement between Gaza and Israel, public radio said.
Read: Speculating endgames: What could happen in Gaza
Public radio quoted Major General Sami Turgeman, the senior officer for the Gaza region, as saying that the destruction of militants' remaining tunnels into Israel could be complete "in a few days".
More than 100 Palestinians died in the Gaza Strip Wednesday, among them the victims of Israeli fire on a market and the UN school where Palestinians fleeing the fighting had sought refuge.
At least 17 people were killed in the strike on the market in Shejaiya, near Gaza City, as Israel observed a four-hour humanitarian lull in other parts of the crowded coastal strip.
The market strike came hours after Israeli shells slammed into a UN school in Jabalia refugee camp which was sheltering some 3,300 homeless Gazans, killing 16 and drawing a furious response from the United Nations.
Reprehensible attack: Ban
"This morning a UN school sheltering thousands of Palestinian families suffered a reprehensible attack," UN chief Ban Ki-moon said Wednesday on a visit to Costa Rica.
"It is unjustifiable, and demands accountability and justice."
The attack was also denounced by the White House in a carefully worded statement that avoided mentioning Israel.
"The United States condemns the shelling of a UNRWA school in Gaza, which reportedly killed and injured innocent Palestinians, including children, and UN humanitarian workers," a statement said.
The Pentagon later said it had granted an Israeli request for ammunition, including some from a stockpile stored by the US military on the ground in Israel for emergency use by the Jewish state.
Rights group Amnesty International had urged Washington to halt arms supplies to Israel.
Read: Israeli strikes hit UN school, Gaza shopping area as toll reaches 1,300
"It is time for the US government to urgently suspend arms transfers to Israel and to push for a UN arms embargo on all parties to the conflict," it said in a petition to US Secretary of State John Kerry.
Hamas said Wednesday it fired rockets at Tel Aviv and the southern port city of Ashkelon in response to the market and school strikes.
The Israeli military said that a rocket hit open ground "in the Tel Aviv area" and another two were intercepted over Ashkelon.
It said that a total of 81 rockets fell in Israel on Wednesday, with another nine shot down by missile defences and that Israel hit 88 targets in Gaza.
Watch: Israeli airstrike on busy Gaza market during humanitarian ceasefire
Early Thursday, Israeli warplanes attacked a mosque near the same UN school in Jabalia, wounding 15 Palestinians, emergency services said.
Medics said two more Palestinians died Thursday of wounds sustained previously, bringing the death toll from 23 days of unrelenting Israeli attacks to 1,363.
In Israel, the army said another three soldiers were killed in Gaza, raising the overall number of soldiers killed to 56 since the operation began on July 8.
Read: Israel intensifies Gaza assault, bombs power plant
Israeli team in Cairo
Despite the loss of life, there appeared to be little Israeli appetite for a truce, with a senior official telling Haaretz newspaper that the Jewish state was not even close to a ceasefire.
"When a ceasefire proposal that answers Israel's important needs is laid on the table, it will be considered," he said, warning that the military operation would expand.
"The (military) will expand attacks against Hamas and the rest of the terror organisations."
Nevertheless, a two-member Israeli delegation travelled to Cairo late Wednesday to discuss a possible ceasefire with Egyptian officials, an official at the airport told AFP, saying they were expected to leave after several hours.
Cairo, a key mediator in previous truce negotiations between Israel and Hamas, was also expected to host a Palestinian delegation later this week.
Full coverage: Gaza under attack