Israel's defense minister said on Friday that the campaign against Hezbollah was not yet complete, just hours after a 10-day ceasefire came into force in Lebanon.
He also warned that if the fighting resumed, displaced residents returning to the country's war-torn south would have to evacuate again.
"The ground maneuver into Lebanon and the strikes on Hezbollah have achieved many gains, but they are still not complete," Defense Minister Israel Katz said in a broadcast statement.
There remained areas of the south that had not yet been cleared of Hezbollah militants, which would have to happen one way or another, he added.
"The area between the security zone and the Litani (River) line, which is currently under our control, has not yet been cleared of terrorists and weapons," Katz warned.
"This will have to be carried out either through diplomatic means or by continued IDF activity once the ceasefire ends."
As the truce took effect at midnight (2100 GMT), thousands of displaced Lebanese civilians began heading south, hoping to return to their homes.
But Katz said a fresh bout of fighting could force them to leave again.
"If the fighting resumes, those residents who return to the security zone will have to be evacuated to allow completion of the mission," AFP quoted him as saying.
According to details of the truce released by the US State Department, Israel reserves the right to continue targeting Hezbollah to prevent "planned, imminent, or ongoing attacks".
Israel, whose troops are occupying parts of southern Lebanon, has also said it will maintain a 10-kilometer (six-mile) security zone it has established along the border.
"The security zone has been cleared of militants and weapons, is empty of residents, and will continue to be cleared of terrorist infrastructure, including the destruction of homes in front-line villages that have effectively become terrorist outposts," Katz said.
The details of the truce also stipulate that Lebanon "with international support... will take meaningful steps to prevent Hezbollah" from undertaking any attacks against Israeli targets.
