GAZA CITY (CNN) -

A rocket was fired into northern Israel from southern Lebanon, and the Israeli military responded with an artillery strike, a spokesman for the Israel Defense Forces said Friday.

The rocket landed near the northern Israeli town of Metula, which sits right by the Lebanese border, and no damage or injuries have been reported. It was not immediately clear who fired the rocket.

The IDF spokesman said that Israel holds the Lebanese government responsible for the attack.

Two rockets were launched from the town of Mari in southern Lebanon, Lebanon's official news agency NNA reported, while a third rocket failed to launch.

The artillery strike from Israel landed on the outskirts of Kfarshouba, the news agency added.

The exchange of fire comes amid Israel's aerial offensive in Gaza against the militant Palestinian group Hamas and the stream of rockets fired into Israel from Gaza.

Israel calls up reservists

As fears of an Israeli ground assault grew among Gaza residents on Thursday, Israel revealed it has beefed up its forces by calling about 30,000 reservists to their units.

"We are utilizing that force to enable us to create a substantial force around Gaza, that if it is required, we'll be able to mobilize as soon as possible," Israel Defense Forces spokesman Peter Lerner told CNN's Wolf Blitzer.

The Israeli Cabinet has authorized the military to call up 40,000 troops if needed. That is 10,000 more than were called up during Israel's offensive into Gaza in November 2012.

Government spokesman Mark Regev said units have been deployed.

"We're ready to go, if we need to go," he told Blitzer.

Regev said Israel didn't want a situation where Hamas, which controls Gaza, was given a "timeout" where it could regroup before restarting its attacks.

The mood is grim in Gaza, where Israeli airstrikes have killed at least 91 Palestinians, including women and children, and injured more than 620 since they began late Monday, Palestinian officials said. They said 23 people died in strikes Thursday and four more people succumbed to their injuries.

Throughout Gaza, people are expecting an Israeli ground incursion. Many people have nowhere to flee and there are no bomb shelters.

"I can't leave. I have nowhere to go. Better to stay at home, inside and be safe," one resident of the town of Bait Hanoun in northern Gaza told CNN.

Most residents live in homes without safe rooms and walls made of breeze block, which a bullet could pass through like paper.

Rising death toll

Each day the death toll has risen in Gaza. The Israel Defense Forces said Thursday it has struck at 785 Hamas targets since announcing the start of its offensive Monday with the aim of hurting Hamas and stopping rocket attacks on Israel.

Tensions in the region flared last week after the killings of three Israeli teenagers in the West Bank, followed by the killing in Jerusalem of a Palestinian teenager that police say may have been an act of revenge. Israel blames Hamas for the deaths of the three Israeli youths, although the group has denied responsibility.

The IDF has said its targets in Gaza include rocket launchers, tunnels and the homes of senior Hamas leaders, which the IDF describes as "command centers."

But among the dead are 22 children and 15 women, including an 18-month-old baby and an 80-year-old woman, according to information from the Palestinian Health Ministry.

The Palestine Liberation Organization said Israeli bombs have hit civilian infrastructure, including a line that provides water to a refugee camp and a sewage plant.

The IDF has not responded to the accusations. It says it uses phone calls and drops empty shells on roofs -- what it calls "roof knocking" -- to warn civilians that airstrikes are imminent. But the approach doesn't guarantee their safety.

In one case, members of a family returned to a house in Gaza shortly after having been warned to evacuate it, Lerner, the Israeli military spokesman, said. They were caught in the airstrike.

He called their deaths a tragedy, saying, "This is not what the IDF does."

U.S. willing to help broker cease-fire

Hopes for a cease-fire appeared dim even as world leaders called for the two sides to stop the violence.

U.S. President Barack Obama and Israeli Prime Minister Benjamin Netanyahu spoke by phone.

"The United States remains prepared to facilitate a cessation of hostilities, including a return to the November 2012 cease-fire agreement," the White House said in a written statement, referring to the Egyptian-brokered deal that halted the previous Israel-Hamas conflict.