The humanitarian organisation said the decision was necessitated by intensifying military operations in Gaza City.
The army said most of those detained were released following on-site questioning.
Half a million people in central Gaza are at risk of losing access to medical care in the coming hours.
The current conflict began on October 7, 2023, when the Palestinian Islamist Hamas militia and others invaded Israel, killing 1,200 and taking some 250 hostage.
OCHA spokesperson Jens Laerke, in a statement on Tuesday, rejected any suggestion to stop the lifesaving aid operation.
Mr Guterres earlier called for a pause in hostilities in the isolated area to allow for the polio vaccination of hundreds of thousands of children.
The seven WCK staff members died on April 1, when Israel Defence Forces drones targeted three vehicles carrying them.
“These are people who are angels. I served alongside them in Ukraine, Gaza, Turkey, Morocco, the Bahamas, and Indonesia.’’
One Israeli strike on the Jabalia refugee camp in northern Gaza on Sunday killed 90 Palestinians, Gaza’s health ministry spokesperson said.
