But military officials denied this heralded a new phase in Israel’s offensive against Hamas militants.
Earlier, PM Ehud Olmert said Israel was nearing its military goals and operations would go on.
Israel says it carried out 12 overnight airstrikes. One rocket attack was reported from Gaza on Monday morning but there were none overnight.
Previous nights have seen as many as 60 pre-dawn Israeli strikes.
Reports suggest diplomatic efforts between Egypt and Hamas in Cairo are progressing, as envoy Tony Blair heads to the city for talks on Monday.
Nearly 900 Gazans have been killed during the conflict, Palestinian medics say. Israel says 13 Israelis have died.
Nearly 40 people were killed across Gaza on Sunday, Palestinian sources said – 17 in Gaza City, as Israel’s troops reportedly engaged in fierce fighting there.
Israel is preventing international journalists from entering the coastal strip, making it impossible to independently confirm such figures.
Israel hopes the scale of its operation will greatly reduce the number of missiles fired from Gaza into southern Israel, while eroding support for Hamas.
Militants fired more than 20 rockets on Sunday, slightly injuring three people, but no more rockets were fired overnight.
Confirming the deployment of reserve soldiers, Israeli government spokesman Mark Regev told the BBC reservists had been called up “a few days back” to augment its forces.
“We’re keeping the military pressure up on Hamas, we think our pressure has been effective and continues to be effective in taking apart their military machine,” he said.
The Israeli military said some reservists were being used to refresh troops currently in action in Gaza, but that this did not yet constitute an escalation of the campaign.
Brig Gen Avi Benayahu, Israel’s chief military spokesman, said thousands more – who are to comprise a new, expanded phase in the ground operation – were still in training and had not been deployed.
On Sunday Israel dropped new leaflets into Gaza and left phone messages warning Gazans to stay away from areas used by Hamas, saying its operation would soon enter “phase three”, the Associated Press reported.
In Cairo, talks between Hamas and Egyptian intelligence chief Omar Suleiman were described by an unnamed intelligence official as “positive”, the state news agency reported, without providing details.
Former UK Prime Minister Tony Blair, now Middle East envoy for the Quartet – the US, EU, UN and Russia – is due to meet Egyptian President Hosni Mubarak in Cairo on Monday morning.
Gaza facing ‘medical crisis’
On Sunday, after an Israeli cabinet meeting in Jerusalem to consider the country’s next move, Mr Olmert praised the military’s “impressive gains” in Gaza and said it was time to “translate our achievements into the goals we have set”.
“Israel is nearing the goals which it set itself, but more patience, determination and effort is still demanded.”
Referring to last week’s UN Security Council call for an immediate ceasefire, Mr Olmert said “nobody should be allowed to decide for us if we are allowed to strike”.
Both Hamas and Israel have rejected the UN resolution.
In Gaza the main hospital is close to collapse, according to two Norwegian doctors who have been working there during the conflict.
They said patients at al-Shifa hospital are dying because of a lack of specialist doctors and basic medical equipment.
Doctors Mads Gilbert and Erik Fosse said half of their patients were civilians, some of them young children with shrapnel and blast wounds.
They told the BBC that 12 ambulance staff had been killed in shelling, despite their clearly-marked vehicles.
Frequent power cuts mean surgeons are having to perform some operations by torchlight, they said.
“I think we could sum it by saying that it’s been a living hell for the Palestinians,” said Dr Gilbert.
Aid agencies say Gaza’s 1.5 million residents are in urgent need of food and medical aid.
Meanwhile, Israel’s army denied deploying white phosphorus bombs in Gaza, after Palestinian medics said they had treated patients for burns caused by the munitions.
Israel began Operation Cast Lead just weeks before parliamentary elections in the country, as a six-month truce with Hamas unraveled.
BBC News, 12 January 2009