As Israel embarked on a large-scale civil defense exercise on Sunday, Prime Minister Benjamin Netanyahu sought to reassure Israelis and some jittery Arab neighbors that the nationwide drill was not meant to signal a deterioration in security or an imminent war.

“This is a routine exercise that has been scheduled for some time,” Mr. Netanyahu said at the start of the weekly cabinet meeting. “I would like to make it clear that it is not the result of any exceptional security development. On the contrary, Israel aspires towards calm, stability and peace.”

The five-day exercise, designed to test the readiness of citizens, the emergency services and the local authorities in the case of war, is taking place for the fourth consecutive year. It comes amid growing concern in Israel about the rocket and missile capabilities of militant groups on its borders, and the potential threat of a nuclear Iran.

But the exercise appears to have rattled nerves in Lebanon, where Israel fought a monthlong war against Hezbollah, the Iranian-backed militia, in 2006. Lebanon’s prime minister, Saad Hariri, told reporters in Cairo on Saturday that “to launch military exercises at such a time runs counter to peace efforts” with the Palestinians.

Hezbollah’s deputy head, Nabil Qaouk, said Friday that the military exercise was a sign of Israel’s aggressive intentions and that Hezbollah had gone on alert. Mr. Qaouk was speaking during a meeting at his home in southern Lebanon with the Jewish American intellectual Noam Chomsky, a fierce critic of both American and Israeli policy who was barred by Israel from entering the occupied West Bank from Jordan last week.

The Israeli military said the drill will include the sounding of sirens throughout Israel on Wednesday and will “replicate emergency scenarios” in more than 30 local authorities, in cooperation with the Home Front Command.

In Gaza, unidentified gunmen attacked the site of a United Nations children’s summer camp before dawn on Sunday, burning empty water tanks and plastic sheds.

The Hamas government that rules the Palestinian enclave condemned the attack and said it had opened an investigation.

The beachside camp, which was still under construction, was set up by the United Nations Relief and Works Agency, which provides assistance to Palestinian refugees and their descendants. It was scheduled to open next month.

More than 20 masked militants were said to have taken part in the attack. The camp’s guard, Ibrahim Eliwa, 37, said he was awake at 3 a.m. when he saw “an army of gunmen approaching the camp.”

Mr. Eliwa was handcuffed. He said the gunmen tucked an envelope into his coat containing a letter and three bullets. “The letter carried threats to senior staff officials at U.N.R.W.A.,” he said.

John Ging, operations director of the United Nations agency, promised to rebuild the camp and said that the agency would not be intimidated.

“It is a disgraceful situation,” he told reporters in Gaza. “There is no doubt in my mind that it is vandalism linked to a certain degree of extremism.”

The agency has been running summer programs for the past five years for about 250,000 children who study in its schools in Gaza. Hamas, the Islamic militant group, has been running its own camps since it took over Gaza in 2007.

In the Hamas camps, strict, bearded men, sometimes waving sticks, teach children the basic tenets of Islam. In the more popular United Nations camps, children’s activities have included painting, singing and swimming.

Last summer, Younis al-Astal, a Hamas lawmaker, accused the United Nations agency of “implementing a plan to spoil the growing generation of Gaza.”