He said that the attack was carried out by people with an "extremist mentality" and vowed to continue his agency's work in Gaza.
No one claimed the attack, but it appeared to have been carried out by Muslim groups who may be opposed to the camps because they allow boys and girls to mingle.
Gaza's police force was investigating the incident, and Ihab al-Ghussein, the interior ministry spokesman, said "the action is strongly condemned and those behind it will be held accountable".
Our correspondent said that although this was the first time the camp had been attacked, there was concern that parents may not send their children to the camp due to safety concerns.
"Usually around 250,000 children attend this camp over summer. This one is the biggest of about four in Gaza," she said.
The Gaza Strip has been administered by Hamas since June 2007, when it took control of the territory after routing forced loyal to Mahmoud Abbas, the Palestinian president.