Israeli airstrikes continued on Monday as the country’s military called up about 300,000 reservists ahead of a potential ground invasion of Gaza.

Fighting between Israeli soldiers and Islamist Hamas militants continued in the border area with Gaza on Monday, as Israel’s defence minister vowed a “complete siege” of the coastal strip.

United Nations Secretary General, António Guterres urged restraint, acknowledging “Israel’s legitimate security concerns” but expressing anguish that the “extremely dire” conditions for civilians in Gaza “will only deteriorate exponentially.”

Israeli leaders, however, vowed massive and lengthy retaliation for the deadly attacks on Israel launched by Hamas from the Gaza strip on Saturday.

Israeli gunships also launched strikes in southern Lebanon, raising fears in the region that the conflict could broaden and escalate further.

Prime Minister Benjamin Netanyahu said Israel’s response “will change the Middle East.”

“What Hamas will face will be harsh and terrible,” Netanyahu said on Monday during a meeting with officials from towns in southern Israel, according to a statement. “We are only at the beginning.”

He called on the opposition to enter an emergency government.

“We are engaged in a mission for our homeland, a war to secure our existence, a war that we will win,” he said in a televised speech.

Netanyahu declared the divisions among Israelis to be over. “We are all united.”

At least 900 Israelis were killed and 2,600 wounded in the Hamas attacks on Saturday in what was the worst civilian bloodbath in the country’s history, according to the latest count by Israeli authorities.

More than 4,500 rockets have since been fired at Israel, according to official figures.

Repeated Israeli strikes on densely populated Gaza, meanwhile, killed at least 687 people and left more than 3,800 injured, according to the health ministry there.

Hamas militants took at least 150 people captive and brought them back to Gaza, including women, children and the elderly.

In addition to Israelis, citizens of a number of other countries are believed to be among the captives, NAN reports.