Left-wing NGO B'Tselem released a statement condemning the execution of more than 20 people in Gaza by Hamas in a matter of days.
The men were publicly executed for being "collaborators", a term used to describe anyone thought to have cooperated with Israel.
The killings followed the assassination of three senior Hamas leaders on Thursday in an Israeli Air Force strike, which is said to have seriously shaken the Islamist terror group and triggered something of a witch hunt to discover how Israel had obtained precise information on their whereabouts at a time of war.
B'Tselem claimed that according to the data available to them, since last Thursday (21 August) - the same day the Hamas leaders were killed - 25 people had been executed by Hamas, seven of whom were shot publicly in Gaza City's central square.
It is unclear what kind of a trial, if any, the accused received.
In a statement released Sunday, B'Tselem said the "current circumstances prevailing in the Gaza Strip" prevented it from carrying out a full investigation, including the precise number of victims or their identities.
In any event, the statement continued, "International humanitarian law totally forbids any state or organization from executing people without a trial... Acts like these are a severe violation of international law."
The statement went on to condemn the death penalty as "immoral" under any circumstances, and called on Hamas to use other methods to punish wrongdoers.
B'Tselem's statement follows calls by a right-wing NGO for the US to take action over the spate of killings.
The Legal Forum for Israel submitted a letter to Washington's Ambassador to Israel, requesting "that your government act to put an end to the wave of executions in Gaza."