Mr Brown made the remarks after meetings with French President Nicolas Sarkozy and German Chancellor Angela Merkel at which the situation in Gaza was discussed.
"Today's attack on the UN headquarters in Gaza is indefensible," he said.
"The UN's mission in Gaza is purely humanitarian, bringing relief to civilians suffering in appalling conditions as a result of the ongoing military action.
"UN staff are working on behalf of the international community - any attack on them is unacceptable as Israel has acknowledged.
"The intensification of Israeli military action, and continued Hamas rocket attacks, reinforce the urgency of our call for an immediate ceasefire."
In the House of Commons, foreign minister Bill Rammell described the incident as "unacceptable".
Speaking later to UN general secretary Ban Ki Moon, Mr Brown reiterated the UK's "unwavering support" for international efforts to bring an end to the violence on both sides.
The PM said he believed proposals to end the smuggling of weapons into Gaza from Egypt and effective monitoring of border crossings - the subject of intense negotiations in recent days - could be "accepted" by all sides.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.