Fears of a ground invasion into the Hamas-controlled Gaza Strip continued Thursday as Israel began moving troops and armoured tanks towards the Palestinian territory, while Hamas fired rockets at Tel Aviv, sparking the first air-raid sirens in the city since the 1991 Gulf War.

It is unknown where the two rockets aimed at Tel Aviv landed, raising the possibility they fell into the Mediterranean. A third rocket landed in an open area on the southern outskirts of the city.

Defence minister Ehud Bark announced he had authorized the army to draw from reserve forces to possibly enter Gaza, but military officials said no decision had been yet. As many as 30,000 additional troops could be drafted.

At least 12 trucks were seen transporting tanks and armoured personnel carriers toward Gaza late Thursday, along with several buses carrying soldiers.

Israeli media said the military was expected to enter Gaza Friday, however, military officials did not confirm the reports.

“In the past 24 hours Israel has made it clear that it will not tolerate rocket and missile attacks on civilians,” Israeli Prime Minister Benjamin Netanyahu said during a press conference Thursday. “I hope that Hamas and the other terror organizations in Gaza got the message.”

Following Israel’s airstrike on Gaza Wednesday, Hamas responded by sending hundreds of rockets into southern Israel in less than 24 hours.

By nightfall Thursday, Hamas said it had fired more than 350 rockets into Israel.

Israeli military officials estimate that Gaza militants have as many as 12,000 rockets, said some 220 rockets struck the Jewish state and another 130 were intercepted by the country’s missile defence system, the Iron Dome.

Rockets rained on Kiryat Malachi, a southern Israeli town of 20,000, which led to the deaths of three civilians and forced the shutdown of schools and factories.

The deaths in Kiryat Malachi were the first in Israel since it launched its operation Wednesday and raised the prospect of even fiercer Israeli retaliation. Israel has been targeting rocket stores in Gaza and has reportedly destroyed dozens of launchers.

The Israeli airstrikes struck dozens of sites across Gaza. A total of 18 Palestinians, including seven civilians, have been killed and more than 100 people wounded, according to Palestinian medical officials. Among the dead were three children.

The violence that has erupted between Israel and Hamas has been described as the worst seen in four years.

In Israel’s commercial capital Tel Aviv, the air-raid sirens were sounded for the first time in two decades.

Israel declared a state of emergency in the country's south, where more than one million Israelis live within rocket range, and residents were instructed to remain close to fortified areas.

“For my children is terrible,” said Kiryat Malachi resident Sharon Dalisi.

He said he is planning to take is children out of the area until the violence settles down.

In Gaza, 33-year-old mother of three Zainab Nimr told the Associated Press she had enough food and water for four days but she fears that the violence could continue for much longer.

“I am trying to calm my children when they hear the sound of explosions,” she said.

Israeli Ambassador to Canada Miriam Ziv told CTV’s Power Play on Thursday that the attack on Gaza began after days of continued shelling on southern Israel.

“Now the southern part of Israel will have about a million citizens, more or less the size of Ottawa. Now just imagine Ottawa is being shelled,” she said. “We felt this was the moment we should retaliate and react because we have a legitimate right as a country to defend our citizens.”

The attack led to the death of Ahmed al-Jabari, head of Hamas’s military arm.

“The blood of many Israelis is on his hands,” said Ziv.

Al-Jabari is the most senior Hamas official to be killed since early 2009, when the last major conflict between the two sides ended. Israel considered al-Jabari to be responsible for a number of fatal attacks, as well as the kidnapping of Israeli soldier Gilad Schalit in 2006. Schalit was freed last October in a swap of more than 1,000 Palestinian prisoners.

Ziv said more powerful weapons have been smuggled into Gaza in recent years via dozens of underground tunnels.

“Iran has been sending arms all the time to Gaza throughout the years,” she said. “Recently the sophistication of the arms smuggled into Gaza has grown.”

The United States came out in support of Israel on Thursday, with White House spokesman Jay Carney saying there is "no justification" for rocket fire from Gaza and urging militants to stop "cowardly acts."

Canada’s Foreign Affairs Minister John Baird spoke by telephone Thursday with his Israeli counterpart, Avigdor Lieberman.

Baird affirmed Israel's right to defend itself from a "terrorist act," spokesman Rick Roth said in an email.

"The minister was saddened to hear that more than one million Israelis have been forced into shelters today. Baird concluded by telling Minister Lieberman that Israel is in our thoughts and prayers."

Baird's office declined to say whether he urged Israel to avoid inflicting civilian casualties.

"I think that everyone recognizes that the loss of any innocent life is extremely unfortunate in these situations. We will continue to monitor the situation very closely and will make appropriate representations when needed," said Roth.

Elliot Tepper, a professor of international relations at Carlton University, told CTV News Channel that it is a “delicate” time in the Middle East and the escalation into an all-out war is entirely possible.

“That Egyptian initiative could possibly defuse the situation. On the other hand these things have a way of spiraling out of control,” he said “There’s also the possibility that there are others who would like it to go on and on, not only inside Gaza but outside.”

The Muslim Brotherhood called for a “Day of Rage” in Arab capitals to protest Israel’s actions, which Tepper said will only flare tensions.

“Iran has a way to play around here in a mischievous way so the issue of control is key here and where is the will and where is the control,” said Tepper.

With files from The Associated Press, The Canadian Press and a report from CTV News Middle East Bureau Chief Martin Seemungal