GAZA CITY, Gaza Strip — Israeli tanks and troops launched a ground offensive in the Gaza Strip Saturday night with officials saying they expected a lengthy fight with Hamas militants in the densely populated territory after eight days of punishing airstrikes failed to halt rocket attacks on Israel.

Hamas vowed that Gaza would be a “graveyard” for Israelis forces.

“This will not be easy and it will not be short,” Israeli Defense Minister Ehud Barak said soon after the ground invasion began.

The incursion was preceded by several hours of heavy artillery fire after dark, igniting flames in the night sky. Machine gun fire rattled as bright tracer rounds flashed through the darkness and the crash of hundreds of shells sent up streaks of fire.

Artillery fired illuminating rounds, sending streaks of bright light drifting down over Gaza’s densely packed neighborhoods. Gunbattles could be heard, as troops crossed the border into Gaza, marching single file. They were backed by helicopter gunships and tanks.

Israeli security officials said the objective is not to reoccupy Gaza. The depth and intensity of the ground operation will depend on parallel diplomatic efforts, the officials said on condition of anonymity because they were not authorized to speak to the media.

“I don’t want to disillusion anybody and residents of the south will go through difficult days,” Barak said. “We do not seek war but we will not abandon our citizens to the ongoing Hamas attacks.”

The U.N. Security Council scheduled emergency consultations Saturday night on the escalation of violence in Gaza.

Eight days of airstrikes have left more than 460 Palestinians dead and four Israelis were killed by rocket fire. Gaza is densely populated, and intense urban warfare in those conditions could exact a much higher civilian toll.

The U.N. estimates that at least a quarter of the Palestinians killed so far were civilian.

“We have many, many targets, Israeli army spokeswoman Maj. Avital Leibovich told CNN. “To my estimation, it will be a lengthy operation.”

Before the ground incursion began, heavy Israeli artillery fire hit east of Gaza City in locations were Hamas fighters were deployed. The artillery shells were apparently intended to detonate Hamas explosive devices and mines planted along the border area before troops marched in.

“Gaza will not be paved with flowers for you, it will be paved with fire and hell,” Hamas warned Israeli forces. Spokesman Ismail Radwan said in a televised speech Gaza will “become a graveyard” for Israeli soldiers.

A text message sent by Hamas’ military wing, Izzedine al-Qassam, said “the Zionists started approaching the trap which our fighters prepared for them.” Hamas said it also broadcast a Hebrew message on Israeli military radio frequencies promising to kill and kidnap the Israeli soldiers.

“Be prepared for a unique surprise, you will be either killed or kidnapped and will suffer mental illness from the horrors we will show you,” the message said.

Hamas has also threatened to resume suicide attacks inside Israel.

Hamas has long prepared for Israel’s invasion, digging tunnels and rigging some areas with explosives. At the start of the offensive, Israeli artillery hit some of the border areas, apparently to detonate hidden explosives.

The Israeli government said tens of thousands of reserve soldiers are being mobilized as the offensive in Gaza widens. Before the ground incursion began, defense officials said about 10,000 Israeli soldiers had massed along the border in recent days.

The offensive began last Saturday with a week of aerial bombardment of Hamas targets, in an attempt to halt Hamas rocket attacks that were reaching farther into Israel than ever before.

Despite the military onslaught, Hamas kept firing at Israeli towns, and Israeli officials said diplomatic efforts did not produced a satisfactory plan so far to guarantee a halt to rockets.

Israel initially held off on a ground offensive, apparently in part because of concern about casualties among Israeli troops and because of fears of getting bogged down in Gaza.

Israeli Foreign Minister Tzipi Livni said before the ground operation began that Israel might have no choice but to move in on the ground.

“There are targets that can be done from the air and targets that cannot,” Livni told Channel 2 TV.

She said Israel had broader objectives than just trying to stop Gaza militants from firing rockets into the country’s south. She told the interviewer that toppling Hamas was “a strategic Israeli objective” but said that more than one military offensive might be needed to achieve Israel’s aims.

“I cannot accept a state controlled by a terror organization in Gaza,” Livni said.

Israeli airstrikes intensified just as the ground operation was getting under way, and 28 Palestinians were killed. One raid hit a mosque in the northern Gaza town of Beit Lahiya, killing 13 people and wounding 33, according to a Palestinian health official.

One of the wounded worshippers, Salah Mustafa, told Al-Jazeera TV from a hospital that the mosque was packed.

“It was unbelievably awful,” he said, struggling to catch his breath.

It was not immediately clear why the mosque was hit, but Israel has hit other mosques in its air campaign and said they were used for storing weapons.

Artillery fire is less accurate than attacks from the air using precision-guided munitions, raising the possibility of a higher number of civilian casualties.

An artillery shell hit a house in Beit Lahiya, killing two people and wounding five, said members of the family living there. Ambulances could not immediately reach them because of the resulting fire, they said.

Resident Abed al-Ghoul said the Israeli army called by phone to tell them to leave the house within 15 minutes.

Palestinian militants kept up their fire as well, launching 29 rockets into Israel Saturday, hitting four houses and lightly wounding three people.

One rocket scored a direct hit on a house in the southern city of Ashkelon and another struck a bomb shelter there, leaving its above-ground entrance scarred by shrapnel and blasting a parked bus.

The ground operation sidelined intense international diplomacy to try to reach a truce. French President Nicolas Sarkozy was the visit the region next week, and U.S. President George W. Bush and U.N. chief Ban Ki-moon both spoke in favor of an internationally monitored truce.

Israel has already said it wants international monitors. It is unclear whether Hamas would agree to such supervision, which could limit its control of Gaza. Hamas has ruled the area since seizing control in June 2007.

In Hamas’ first reaction to the proposal for international monitors, government spokesman Taher Nunu said early Saturday that the group would not allow Israel or the international community to impose any arrangement, though he left the door open to a negotiated solution.

“Anyone who thinks that the change in the Palestinian arena can be achieved through jet fighters’ bombs and tanks and without dialogue is mistaken,” he said.