Lebanon's defence minister says anyone firing rockets at Israel from the south will be considered a traitor and be firmly dealt with by the army.
Arab states are discussing reconstruction aid for Lebanon
Elias Murr's remarks are being seen as a warning to militias, but he also expressed confidence Hezbollah was committed to the UN-brokered truce.
Mr Murr was speaking a day after an Israeli raid raised new concerns about the stability of the truce.
But he did not repeat a warning about halting army deployments to the south.
Meanwhile, Israel's military chief, Dan Halutz, has said that the conflict ended in an Israeli victory.
Mr Murr told a Beirut news conference that any ceasefire violation that would give Israel the justification to strike Lebanon would be "treated harshly".
"It will be considered as direct collaboration with the Israeli enemy," Mr Murr said, adding that those responsible "will be tried and referred to a military tribunal".
But he said Hezbollah was "totally committed" to the ceasefire, which came into effect on Monday to end a month of conflict between Israel and Hezbollah militants.
Mr Murr's remarks seemed to carry two messages, says the BBC's Nick Childs in Beirut.
Firstly, that the government was really concerned about the fragility of the ceasefire and secondly that the Lebanese authorities were fulfilling their truce obligations a day after the Israeli commando raid deep inside Lebanon.
However, Mr Murr did not repeat an earlier Lebanese warning that it might halt the deployment of its army in the south in response to the Israeli raid.
Meanwhile, an Israeli panel of inquiry into the way the military campaign in Lebanon was handled has held its first meeting.
Defence Minister Amir Peretz said Israel had to examine its failures.
Some Israeli forces have pulled back to the border
However on Sunday, military chief of staff Dan Halutz reportedly gave a positive assessment of the operations as he addressed a cabinet meeting.
"Tallying up the points, it is definitely a victory, perhaps not a knockout, but in terms of achievements, it is [a victory]," Mr Halutz was quoted as saying by participants in the meeting.
Arab League foreign ministers met in Cairo on Sunday to discuss the Lebanon conflict.
Several promised funds to help Lebanon with reconstruction and some called for an urgent Arab League summit which Saudi Arabia has offered to host.
The ministers also called on the UN Security Council to convene a meeting next month to consider an Arab initiative for a Middle East peace agreement based on a previous Arab proposal four years ago that was rejected by Israel.
Syria was not represented at the meeting. Syrian President Bashar al-Assad angered some Arab governments last week when he criticised them for being too inadequate to defeat Israel in battle.
In Lebanon itself, Prime Minister Fouad Siniora said the bloodshed and destruction caused by Israel during the month-long conflict amounted to a crime against humanity.
"This is a criminal act which reflects Israel's hatred to destroy Lebanon and its unity," he said during a visit to Beirut's heavily bombed southern suburbs.
Also on Sunday, a contingent of 150 French soldiers sailed from the French port of Toulon, to join the 50 French soldiers who arrived in Lebanon a day earlier.
The new contingent is to be deployed "close to the Israeli border," their commander, Colonel Christophe Issac, told AFP news agency.
The UN force and the deployment of Lebanese government troops to the south of the country, where Hezbollah have been active, is one of the conditions of the ceasefire.
The UN wants 3,500 troops on the ground by the end of August, to be increased later to 15,000.
It says it is disappointed with the French contribution and wants other European nations to offer more help too.
On Sunday France called for a European Union meeting next week to determine how member countries planned to support the UN force.
"We ask that European solidarity is expressed as soon as possible about Lebanon," French Foreign Minister Philippe Douste-Blazy said.
Meanwhile, UN Secretary General Kofi Annan expressed deep concern over Saturday's Israeli raid in the Bekaa Valley, calling it a ceasefire violation.
Israel said it was trying to disrupt weapons supplies from Iran and Syria to Hezbollah, and insisted the truce held.
The raid left one Israeli dead. Hezbollah denies reports it lost three militants.