Major Avital Leibovich, an Israeli military spokeswoman, told Al Jazeera that "a few reserve units have entered Gaza to participate in the operation" but would not say how many soldiers were involved.
"We are not talking about a massive amount of forces, rather a limited one," she said.
Al Jazeera's Alan Fisher, reporting from the Israel-Gaza border, said the move appeared to be in preparation for the so-called third stage of Israel's offensive: moving troops into the towns and cities of Gaza to fight door-to-door.
"The next 48 hours will be fairly crucial in what is happening in Gaza," he said.
"It's clear that the Israelis are using their military strength to try to bring Hamas to the negotiating table to agree to some sort of ceasefire, to some sort of truce.
"With the call up of reserves, what we will see is some sort of intensification of the effort inside Gaza itself, targeting Hamas buildings, Hamas infrastructure, even Hamas operatives.
"And at the end of those 48 hours if Hamas haven't made some sort of indication that they are willing to accept a truce, willing to stop firing the rockets, then I think at that stage we will move into stage three which will be a large scale advance into the heart of the towns and cities of Gaza."
The deployment came amid some of the most intense fighting since the Israeli ground offensive began on January 3, with Palestinian fighters putting up stiff resistance to the Israeli advance into Gaza City.
The total number of Palestinian deaths since Israel began its war on December 27 climbed to 905, about a quarter of them children.
Almost 4,100 Palestinians have also been wounded since the beginning of the offensive.
Thirteen Israelis have been killed during the same time, including three civilians hit by rockets fired from the Gaza Strip into Israel.
Meanwhile, Egyptian authorities allowed 61 Arab doctors to enter the Gaza Strip via the Rafah crossing on Monday morning.
A humanitarian corridor was opened on Monday, with Israel allowing 105 trucks to enter the territory via the Kerem Shalom crossing and another 60 to go through the Kari border point.
Princess Haya bint al Hussein, a UN Messenger of Peace and wife of the ruler of Dubai, told Al Jazeera that efforts to get aid into the Gaza Strip have been hampered: "I heard yesterday that from the 10 trucks we had scheduled to go in, which started at 15, now we've been asked to drop the capacity of each truck by 10 per cent.
"Things like this when the food is ready to go in, sitting on the border really do cause major frustrations.
"I can't understand why we have red tape in humanitarian efforts."
Olmert told an Israeli cabinet meeting on Sunday that "Israel is getting close to achieving the goals it set for itself".
He told ministers that Israel had "dealt Hamas an unprecedented blow... It will never be the same Hamas", according to Oved Yehezkel, the Israeli cabinet secretary.
But the Israeli military onslaught has, so far, failed to achieve the stated aim of stopping Palestinian fighters from firing rockets into southern Israel.
About 20 rockets were fired across the border on Sunday, but did not cause any casualties.
'Third stage' debate
The Israeli cabinet meeting had been expected to include discussion of a possible "third stage" of the offensive in which the military would enter Gaza's urban areas.
|Israeli citizens in Ashkelon shelter from rockets fired from the Gaza Strip [AFP]
However, several Israeli officials suggested that the offensive could be drawing to a close after last week's UN Security Council resolution calling for an immediate ceasefire.
"The decision of the security council doesn't give us much leeway," Matan Vilnai, the deputy defence minister, told public radio.
"Thus it would seem that we are close to ending the ground operation and ending the operation altogether."
Giroa Eiland, a former Israeli national security adviser, told Al Jazeera that there was a debate within the Israeli government and security establishment about what the goals of the operation should be at this stage.
"The main question is how to conclude and accomplish the missions," he said.
"As far as I can understand one of the reasons the military option might be expanded is in order to give an Israeli solution to the situation."
Amos Gilad, a senior Israeli official, was expected to travel to Cairo in the coming days to discuss a plan to end the fighting after Hamas officials met Egyptian officials on Sunday.