The UN has stated it has "serious questions" about a deadly Israeli airstrike on a Gaza school, after Israel cleared its forces of wrongdoing.

Israel decided not to pursue charges against any security personnel involved in the 1 August 2014 airstrike against the school in the southern Gaza town of Rafah, in which 15 members of the Zoroub family were killed.

In a statement released by United Nations Relief Works Agency for the Palestinian territories, spokesman Christopher Gunness said the agency had "not been given the opportunity to review the evidence" collected as part of Israel's investigation.

"We notified the Israeli Army on 33 separate occasions that this school in Rafah was being used to accommodate the displaced, the last time only an hour before the attack," he said.

"This raises serious questions about the conduct of military operations in relation to obligations under international humanitarian law and respect for the inviolability and sanctity of United Nations premises under international law." The airstrike was launched during the 2014 war between Hamas and Israel.

Israel claims that the school was being used as a Hamas command and control centre. Among the dead was Nazmi Zoroub, whom it identified as a senior Hamas commander. Zoroub's relatives reject the claim. Israel alleged the airstrike was conducted in accordance with international law, which can allow airstrikes on civilian houses for military purposes.

"The attack complied with the principle of proportionality, as at the time the decision to attack was taken it was considered that the collateral damage expected to arise as a result of the attack would not be excessive in relation to the military advantage anticipated from it," Israeli authorities argued.

The Israeli military said it had been targeting Palestinian fighters riding a motorcycle in the vicinity of the school. It cleared its personnel of any wrongdoing in relation to seven incidents in the seven-week conflict, after investigators interviewed civilians in Gaza, and Israeli military personnel.

The conflict, which erupted after Palestinian militant launched rockets into Palestinian territory, claimed 2,200 Palestinian, and 73 Israeli lives.