In a brief announcement, it said it was striking Hezbollah targets in areas close to the Israeli border, and that air force and artillery units were carrying out attacks to support the ground forces. It gave no details on how long the operation would last, but said the army had been training and preparing for months.
"A few hours ago, the IDF began limited, localized and targeted ground raids," it said. "These targets are located in villages close to the border and pose an immediate threat to Israeli communities in northern Israel."
The incursion came shortly after it was approved by Israeli political leaders and marked a new stage in Israel's war against Hezbollah, an Iranian-backed militant group.
Earlier, U.S. officials said Israel had launched small ground raids against Hezbollah and sealed off communities along its northern border on Monday as Israeli artillery pounded southern Lebanon.
State Department spokesman Matthew Miller said Israel informed the U.S. about the raids, which he said were described as "limited operations focused on Hezbollah infrastructure near the border."
The sounds of airstrikes were heard throughout Beirut and smoke rose from the capital's southern suburbs, where Hezbollah has a strong presence, shortly after Israel ordered residents of three buildings to evacuate.
There were no reports of direct clashes between Israeli troops and Hezbollah militants, who last engaged in ground combat on Lebanese soil during a monthlong war in 2006.
Israel and Hezbollah have exchanged fire almost every day since the war in Gaza began, displacing tens of thousands of people in Israel and Lebanon. Israel says it will continue to strike Hezbollah until it is safe for families to return to their homes near the Lebanon border. Hezbollah has promised to keep firing rockets into Israel until there is a cease-fire in Gaza.
Hezbollah vowed Monday to keep fighting even after its longtime leader Hassan Nasrallah and other top officials were recently wiped out by Israeli strikes.
The group's acting leader, Naim Kassem, said in a televised statement that if Israel decides to launch a ground offensive, Hezbollah is ready. He said commanders killed in recent weeks have already been replaced.
The man widely expected to take over the top post from Kassem is Hashem Safieddine, a cousin of Nasrallah who oversees Hezbollah's political affairs.
Shortly before the Israeli invasion, the army declared three northern communities near the Lebanese border to be "closed military zones," indicating that the ground operation was imminent.
The army has heavily beefed up forces along the border in recent days, and commanders have said they are prepared to go into Lebanon if ordered to.
Chris Coyle, a resident of northern Israel, said the army had erected gates and checkpoints throughout the region and positioned scores of tanks along the border in recent days. "They're certainly getting ready to go in," he said.
In the nearby Golan Heights, an Associated Press reporter heard Israeli artillery fire and explosions in southern Lebanon. Israeli forces also fired flares into Lebanon.
An AP reporter in the southern Lebanon town of Marjayoun reported sounds of heavy shelling and explosions and occasional airstrikes coming from areas closer to the border.
Israeli strikes in recent weeks have hit what the military says are thousands of militant targets across large parts of Lebanon. Over 1,000 people have been killed in Lebanon in the past two weeks, nearly a quarter of them women and children, according to the Health Ministry.
Early Monday, an airstrike hit a residential building in central Beirut, killing three Palestinian militants, as Israel appeared to send a message that no part of Lebanon is out of bounds.
Hezbollah's capabilities are unclear after a series of major blows
Hezbollah has significantly increased its rocket attacks in the past week to several hundred daily, but most have been intercepted or fallen in open areas. Several people have been wounded in Israel. There have been no fatalities since two soldiers were killed near the border on Sept. 19.
But Hezbollah's capabilities remain unclear.
As recently as two weeks ago, a strike like Monday's in central Beirut - outside of the main areas where Hezbollah operates and next to a busy transportation hub normally crowded with buses and taxis - would have been seen as a major escalation and likely followed by a long-range Hezbollah strike into Israel.
But the unspoken rules of the long-running conflict no longer seem to be in effect.
It's possible Hezbollah is holding back to save resources for a bigger battle. But the militant group might also be in disarray after Israeli intelligence apparently penetrated its highest levels.
Some European countries began pulling their diplomats and citizens out of Lebanon on Monday. Germany sent a military plane to evacuate diplomats' relatives and others. Bulgaria sent a government jet to get the first group of its citizens out.
Monday's strike in Beirut killed three members of the Popular Front for the Liberation of Palestine, a small, leftist faction that has not been meaningfully involved in months of fighting between Israel and Hezbollah. Israel has not claimed the strike but is widely assumed to have carried it out.
Also Monday, Hamas announced its top commander in Lebanon, Fatah Sharif, was killed with his family in an airstrike on the Al-Buss refugee camp in the southern port city of Tyre. The Israeli military confirmed it had targeted him.
Hezbollah began firing rockets, drones and missiles into northern Israel after Hamas' Oct. 7 attack from Gaza into Israel sparked the war in the Palestinian territory.
Israel responded to the rockets with airstrikes in Lebanon, and the fighting has steadily escalated over the past year. The Lebanese government says the fighting may have displaced up to a million people, although the U.N. estimate is around 200,000.
Israel shows little interest in cease-fire calls as it bloodies a longtime foe
The United States and its allies have called for a cease-fire, hoping to avoid further escalation that could draw in Iran and set off a wider war. But Israeli Prime Minister Benjamin Netanyahu has shown little interest, as his country racks up military achievements against a longtime foe.
France, which has close ties to Lebanon, has joined the United States in calling for a cease-fire. French Foreign Minister Jean-Noël Barrot, visiting Beirut Monday, urged Israel to refrain from a ground offensive.
Barrot also called on Hezbollah to stop firing on Israel, saying the group "bears heavy responsibility in the current situation, given its choice to enter the conflict."
Lebanese Prime Minister Najib Mikati, speaking after meeting with Barrot, said the country is committed to an immediate cease-fire followed by the deployment of Lebanese troops in the south, in keeping with a U.N. Security Council resolution that ended the 2006 war but was never fully implemented.
Hezbollah, which boasts tens of thousands of battle-hardened fighters and long-range missiles capable of hitting anywhere inside Israel, has long been seen as the most powerful militant group in the region and a key partner to Iran in both threatening and deterring Israel.
But Hezbollah has never faced an onslaught quite like this one, which began with a sophisticated attack on its pagers and walkie-talkies in mid-September that killed dozens of people and wounded around 3,000 - including many fighters but also many civilians.
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Melzer reported from Tel Aviv, Israel. Associated Press writers Bassem Mroue and Kareem Chehayeb in Beirut; Sam McNeil in Majdal Shams, Golan Heights; Jamey Keaten in Geneva; Geir Moulson in Berlin; and Veselin Toshkov in Sofia, Bulgaria, contributed reporting.