JERUSALEM — Israel started a military operation on Tuesday to expose and thwart offensive tunnels Hezbollah had been building across the Lebanese border, the military said, the first time that Israel has taken open action to combat underground passageways in the north. The effort, called Operation Northern Shield, was aimed at an unspecified number of tunnels in the area of Metula, said Lt. Col. Jonathan Conricus, A spokesman for the Israel Defense Forces. None of the tunnels were ready to be used, he said, and the army was neither asking civilians in the area to evacuate nor calling up reserves. But it declared an area around Metula, in the northernmost reaches of the Galilee panhandle, a closed military zone and said it had “enhanced its presence and readiness” in the north and was “prepared for various scenarios.” Prime Minister Benjamin Netanyahu said the early stages of the operation had already proven successful. “Whoever tries to harm the state of Israel will pay a heavy price,” he said in a statement. He added that Israel would continue to act, “openly and covertly, to ensure the security of Israel.” Brig. Gen. Ronen Manelis, the chief military spokesman, said Israel was prepared for a “broad operation over several weeks.” It was expected to extend beyond the Metula area, along the border. The military also warned Hezbollah and soldiers of the Lebanese Army to stay away from the tunnels, saying their lives were in danger, though the Israeli Foreign Ministry emphasized that the operation was taking place on the Israeli side of the border, within Israeli territory. With the winding down of the civil war in neighboring Syria, Israel appears to have increasingly shifted its focus to Lebanon. Hezbollah, the Lebanese Shiite organization backed by Iran, has been fighting for years against insurgent groups in Syria to defend the rule of President Bashar al-Assad, and Israel has been working intensively to prevent Iran’s efforts to entrench itself in Syria. But Israel has also been warning in recent months of Iranian efforts to strengthen Hezbollah in Lebanon, making a conflagration seem only a matter of time. While Israeli experts said the action against the tunnels could lead to an escalation, it was not immediately clear if, or how, Hezbollah would respond. “Now the ball is in the Hezbollah court,” said Yaakov Amidror, a former Israeli national security adviser and retired general. “They can react and the reaction to their reaction might be devastating,” he told reporters on Tuesday, in an apparent effort to deter Hezbollah. Israeli officials have accused Iran of helping Hezbollah build underground factories in Lebanon to upgrade the militant group’s arsenal of missiles. In addition, Israeli news outlets have reported that Iran has been flying advanced weaponry directly to Beirut, bypassing overland routes through Syria that Israel has repeatedly bombed.