One person has been killed and at least 10 others wounded as Israeli soldiers, stationed at the border line between the Palestinian town of Khan Younis and Israel, opened fire on them, medical sources say.
Witnesses told Al Jazeera that the teenagers entered the disputed area of the "buffer zone", which is 300m along all the Gaza-Israel border, southeast of the Gaza Strip.
Health officials said Anwar Qdeih, 23, was hit in the head by Israeli gunfire after he approached the fence that runs between Israel and Gaza - an area that Israel has long declared a no-go zone for Palestinians.
Anwar was hit in the head by the Israeli gunfire, Reuters news agency quoted medics and one of his relatives as saying.
"Anwar was trying to put a Hamas flag on the fence," his relative Omar Qdeih, who was at the scene, said.
"The army fired three times into the air. Anwar shouted at them 'Jabari is behind you', then they shot him in the head,"
he told Reuters.
Israel killed Ahmad Jabari, the acting military chief of Hamas' armed wing, on November 14, launching an offensive in Gaza with the stated aim of halting Palestinian rocket fire into its territory.
Sami Abu Zuhri, a Hamas spokesman in Gaza, said the incident marked a clear violation by Israeli forces of the terms of the truce.
Hamas "will raise this violation with Egyptian mediators to make sure that it does not happen again," he said.
Al Jazeera's Nicole Johnston, reporting from Gaza City, said a number of farmers reportedly entered the buffer zone to check on their crops.
Johnston said they may have had confused details about the zone as there had been lots of information about the easing of travel restrictions as part of the ceasefire agreement between Hamas and Israel, brokered on Wednesday night.
They may have thought that they can now travel there, she said, adding that the shootings would "send a message" to Palestinians that the buffer zone is still a no-go area.
"The occupation forces opened fire on a group of farmers," Gaza emergency service spokesman Adham Abu Selmiya said.
The teenagers reportedly entered along with the farmers when the Israeli soldiers opened fire on them, our correspondent said.
'Attempt to breach border'
An Israeli military spokeswoman said soldiers fired warning shots into the air to push back about 300 Palestinians who had gathered at different locations along the fence and who were attempting to breach the border.
"After the rioters did not comply, the soldiers responded by firing toward the legs of the rioters," she said.
The Israeli army is able to shoot at people in the buffer zone without entering the area, she said.
Another Palestinians was wounded by Israeli gunfire from the border on Thursday, according to health officials, when the Israeli military said 200 Palestinian "rioters" approached the fence.
Israel's army constantly patrols the border area and says its forces have come under increasing attack this year, with
fighters planting explosive devices and firing an anti-tank missile on at least one occasion.
Wednesday's ceasefire deal ended eight days of fierce fighting that left 163 Palestinians and six Israelis dead.
According to the terms of the accord, both Israelis and Palestinians agreed to stop their hostilities. However, the
brief document said details on access to the tense border zone would be worked out in the days ahead.
Forty minutes after the ceasefire was announced, Nader Abumaghasib, a 15-year-old boy, was killed by an unmanned drone.