The U.N. human rights office condemned Monday’s “appalling deadly violence” by Israeli security forces in Gaza and said it was extremely worried about what might happen later on Tuesday, while calling for an independent investigation.

U.N. human rights spokesman Rupert Colville told a regular U.N. briefing in Geneva that Israel had a right to defend its borders according to international law, but lethal force must only be used a last resort, and was not justified by Palestinians approaching the Gaza fence.