Israel says two soldiers and two civilians, including Mengistu, have been held by Hamas in the Gaza Strip since 2014, when Israel launched a devastating military assault on the blockaded Palestinian enclave.
Local media said that an Israeli reconnaissance plane fired a rocket on a group of Palestinians gathering near a position of Qassam Brigades, the military wing of Hamas movement, in Ashkelon.
"Israeli forces have operated with near complete impunity for so long that unlawful killings and other flagrant violations of worldwide law have become the norm", Ayed Abu Eqtaish, Accountability Program director at DCIP said in the report.
The Gaza Health Ministry confirmed the two deaths.
There have been reports about talks between Palestinian factions, Egypt, the United Nations and Israel to reach a long-term truce in the besieged enclave. Two Palestinians were reportedly killed in the strike. One of the grainy black and white video clips shows two men in a watchtower, one appears to open fire as puffs of smoke burst in front of him.More news: Everton agree to sign Yerry Mina from Barcelona
The military released footage that it said was of the incident.
The cross-border incident came after a delegation of senior Hamas officials visited Gaza to discuss a ceasefire agreement with Israel proposed by the United Nations and Egypt.
Hamas warned Israel that it would pay for the attack, without elaborating. Violence between the Palestinians and Israel has gained momentum late March, when Gazans started marching towards the fenced border with Israel in a series of protests calling for the right of Palestinian refugees to return to their lands in Israel.
Gaza's Islamist rulers Hamas have fought three wars with Israel since 2008.