GENEVA: Israeli air operations have struck residential buildings, including large tower blocks, as well as schools and U.N. buildings across Gaza, resulting in civilian casualties, the United Nations Human Rights chief said on Tuesday, citing information gathered by his office.

"International humanitarian law is clear: the obligation to take constant care to spare the civilian population and civilian objects remains applicable throughout the attacks," Volker Turk said in a statement.

In response to Israeli Defence Minister Yoav Gallant's announcement of a tightened blockade for the Gaza strip, Turk said that "sieges" that endanger the lives of civilians are prohibited under international law.