Israel unleashed more airstrikes on Lebanon on Wednesday and Hezbollah militants fired salvos of rockets into Israel, with the militant group claiming to have targeted the Mossad spy agency's headquarters in Tel Aviv.

The Israeli military has mounted its heaviest airstrikes in a year of conflict this week, targeting Hezbollah leaders and hitting hundreds of sites deep inside Lebanon while Hezbollah has fired barrages of rockets into Israel.

There was no let-up on Wednesday. Israel said its warplanes were currently carrying out extensive strikes in south Lebanon and the Bekaa Valley, a Hezbollah stronghold.

At least 51 people were killed and 223 wounded in Israeli strikes on Lebanon on Wednesday, Lebanon's Health Minister Firass Abiad told a press conference.

Israel's military chief told troops on Wednesday that the airstrikes would continue in order to destroy Hezbollah infrastructure and to prepare the way for a possible ground operation by Israeli forces.

"You hear the jets overhead; we have been striking all day," Gen. Herzi Halevi, chief of the general staff of the Israel Defence Forces told troops on the border with Lebanon, according to a statement from the military. "This is both to prepare the ground for your possible entry and to continue degrading Hezbollah."

The military also said it is calling up two additional reserve brigades to the northern border to carry out operational activities.

Iran-backed Hezbollah has fired hundreds of missiles and rockets at Israel in recent days as months of conflict across the border with southern Lebanon has intensified sharply, raising fears the conflict could further destabilize the Middle East. The UN Security Council was meeting on Wednesday to discuss the conflict.

"Risk of escalation in the region is acute. The best answer is diplomacy, and our co-ordinated efforts are vital to preventing further escalation," said U.S. Secretary of State Antony Blinken, as he began a meeting of ministers of the Gulf Co-operation Council (GCC) in New York.

WATCH | Non-state militant group has significant arsenal, personnel: 

How powerful is Hezbollah? | About That

Israel has shifted its focus to its northern border, striking Hezbollah targets in southern Lebanon. Andrew Chang breaks down the Iran-backed group's power by examining its military capabilities and its political influence, as both sides inch closer towards a full-blown conflict. Images supplied by Getty Images and The Canadian Press.

Missile intercepted, Israel says

Hezbollah said in a statement it had fired a ballistic missile on Wednesday morning targeting the Mossad headquarters "in support of our steadfast Palestinian people in the Gaza Strip … and in defence of Lebanon and its people." Reuters could not independently confirm the type of rocket fired.

Hezbollah blames Israel for the assassination of several military commanders, as well as the explosions of communication devices belonging to its members last week.

The Israeli military said a single surface-to-surface missile was intercepted by air defence systems after it was detected crossing from Lebanon. Spokesperson Nadav Shoshani said he could not confirm what Hezbollah's target was when it fired the missile from a village in Lebanon.

"The result was a heavy missile, going towards Tel Aviv, towards civilian areas in Tel Aviv. The Mossad headquarters is not in that area," he said.

Warning sirens sounded in Tel Aviv and elsewhere in central Israel but there were no reports of damage or casualties.

The strike was the first time in the current conflict that a Hezbollah missile had been sighted above Tel Aviv, generally considered a target with the potential to trigger a sharp escalation in Israeli action.

Over 90,000 displaced in Lebanon: agency

Israel has expanded the zones in Lebanon it has been striking since Tuesday night, with attacks for the first time on the beach resort town of Jiyyeh just south of Beirut and Maaysrah.

The strikes also took place in Bint Jbeil, Tebnin and Ain Qana in the south, the village of Joun in the Chouf district near the southern city of Sidon, and Maaysrah in northern Keserwan district.

Hundreds of thousands of Lebanese have fled their homes and hospitals have filled with the wounded since an intensification of bombing on Monday, when more than 550 people were killed in Lebanon's deadliest day since the end of the 1975-1990 civil war.

At least 90,530 newly displaced people have been reported in Lebanon, including nearly 40,000 in 283 shelters, the International Organization for Migration said on Wednesday.

Britain said Wednesday it is moving some 700 troops to Cyprus to help its nationals leave Lebanon.

Canada and the U.S. have also encouraged their citizens to leave Lebanon while commercial methods of travel were still available.

Foreign Minister Mélanie Joly last week estimated there were some 45,000 Canadian citizens still in Lebanon, about half of them officially registered with the Canadian government.

"Canada is not currently offering assisted departures or evacuations for Canadians in Lebanon and there is never a guarantee the Canadian government will evacuate Canadians in a crisis situation," Global Affairs Canada said in a statement.

"Canadians should not rely on the Government of Canada for assisted departure or evacuation."

On Tuesday, Ottawa said two Canadians have died in Lebanon but offered few other details.

On the other side of the border, authorities said the Galilee region of northern Israel was hit by heavy Hezbollah barrages on Wednesday morning.

In one salvo about 40 rockets were fired. Some were intercepted in mid-air, others struck open areas or penetrated air defences into populated areas, they said.

In the Israeli town of Safed, an assisted living facility was hit but no injuries were reported, the authorities said.