A column of tanks entered the besieged territory though the Beit Hanoun crossing shortly after nightfall on Saturday, as the Israeli cabinet said it had called up about 9,000 reservists as part of its preparations.
Alan Fisher, Al Jazeera's correspondent on the Israeli-Gaza border, said that he had witnessed the movement of tanks and armored vehicles in the area.
Hamas has vowed to defeat the Israeli army following the invasion.
Zeina Awad, Al Jazeera's correspondent in Jerusalem, said that Israeli media was reporting that several Hamas fighters had been killed at the start of the ground offensive.
The Reuters news agency said a senior Hamas official has said that its fighters had killed a number of Israeli soldiers.
Ayman Mohyeldin, Al Jazeera's correspondent in Gaza, said: "Residents in the Beit Hanoun area say they have seen the movement of tanks into the territory.
"There is absolute fear and terror, it is pitch black, power lines have been cut throughout the strip, more than 250,000 people in northern Gaza are without electricity.
"The biggest concern is a ground invasion could result in urban warfare.
"Rockets are being fired from deeper and deeper within Gaza and if Israel's intention is to prevent such attacks how far into Gaza, an area densely populated with civilians, will they need to go? "
Speaking to Al Jazeera, James Denselow, a Middle East specialist from Kings College, London, said: "Hamas operates as an asymmetrical force. It knows its strength and knows indiscriminate rockets can cause alarm.
"This is why the Israeli army has used air strikes to 'soften' the ground followed by an assault at night when low-tech armies such as Hamas find it very hard to fight."
The ground assault came as the death toll of Palestinians climbed past 450 on Saturday as a strike on a northern Gaza town killed at least 11 people, including one child, who were praying in a mosque.
PHOTO CAPTION
Israeli soldiers advance near the border with central Gaza during a ground operation by the Israeli army late January 3, 2009.
Al-Jazeera