By Nidal al-Mughrabi
GAZA (Reuters) - A ceasefire largely held on Sunday along a tense Gaza-Israel border on Sunday following a day of fierce fighting, but Israel remained on high alert and boosted its air defenses in case hostilities resume.
Israel carried out dozens of air strikes in Gaza on Saturday, killing two teenage boys, and militants fired more than 100 rockets across the border, wounding three people in a southern Israeli town.
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The ceasefire, the second between Israel and Gaza's dominant Hamas Islamists to be brokered by Egypt this year after a previous day-long flare-up in May, came into force late on Saturday.
"Everyone understands that unless the situation is defused, we will very quickly be back to another confrontation," U.N. envoy Nickolay Mladenov told reporters at his office in Gaza.
Israel's military said that, after assessing the situation, it was reinforcing its Iron Dome rocket defense batteries in the greater Tel Aviv area and in the south, where thousands of residents spent much of the Jewish Sabbath in shelters.
It also called up a limited amount of reservists to help out its aerial defense command.
Israel said that in the initial hours of the ceasefire militants had fired two rockets across the border, of which one was intercepted by the Iron Dome system. There were no reports of an Israeli counter-attack in Gaza.
Later, two mortar bombs were fired towards Israel, which responded by striking the launch tube, the military said.
Weekly clashes at the Israel-Gaza border have kept tensions at a high for months. More than 130 Palestinians have been killed by Israeli forces during protests at the frontier held every week since March, including a teenager on Friday, Gaza medics said. There have been no Israeli fatalities.
Israel says Hamas has been orchestrating the demonstrations, dubbed The Great March of Return, to provide cover for militants' cross-border attacks. Hamas denies this.
"Our policy is clear - we hit with great might anyone who harms us," Israeli Prime Minister Benjamin Netanyahu told his cabinet on Sunday. "I hope that they (Hamas) have gotten the message. If not, they will yet."
Netanyahu also instructed the military to keep targeting Palestinian squads that launch incendiary helium balloons and kites into Israeli fields from northern Gaza. Israel's military fired twice on such groups, wounding three people.
Israel says it has lost at least 7,000 acres (2,830 hectares) of farmland and forests to a recent surge in fires started by Gaza militants using such balloons and kites rigged with flammable material.
Hamas said border demonstrations, at which Palestinians have been demanding the right to return to land lost when Israel was created in 1948, would continue and that the onus was on Israel to show restraint.
"Let the enemy end its aggression first and then the resistance will stop," Hamas leader Ismail Haniyeh said in a eulogy for Amir al-Namara, 15, and Loay Kheil, 16, who were killed when a half-constructed high rise they were playing in was hit by an Israeli missile.
The Israeli military said the building had been used by Hamas for urban warfare training.
Twelve others, passers-by and visitors of a nearby public garden, were wounded in the attack, one of dozens of Israeli air strikes on the densely populated enclave on Saturday which damaged residential and office buildings, shattered car windows and caused panic among residents.
"He wasn't carrying a rocket. He was just an innocent kid," said Amir's grandfather Waleed al-Namara at the boy's wake. "We want the calm to last, and for them to agree on a solution that will benefit the Palestinian people."
The surge in violence comes as Palestinian hopes for an independent state have dwindled and peace talks remain stalled. Gaza, home to 2 million people, most of whom depend on foreign aid, has been under Israeli economic sanctions for 12 years.
Separately, a Fatah faction militant and his son were killed in a blast in a building in Gaza on Sunday. Police said the man accidentally set off an old Israeli shell he was trying to dismantle.
(Reporting by Nidal al-Mughrabi, Jeffrey Heller and Maayan Lubell; Writing by Jeffrey Heller; Editing by Raissa Kasolowsky and Gareth Jones)