The Belfast Zoo in Northern Ireland said it is reviewing animal security after a group of chimpanzees escaped an enclosure using a branch as a ladder.

A video posted online by a visitor shows a chimpanzee climbing to the top of the enclosure while another sat on top of the wall.

In a statement, the Belfast City Council, which runs the zoo, said one of the chimpanzees left the enclosure on Saturday for about an hour, while two others only climbed up to the wall. All the chimpanzees safely returned to the enclosure on their own.

"This was a highly unusual event, despite the natural curious nature of chimpanzees, and Council would like to reassure the public that Belfast Zoo is fully committed to providing a safe and enjoyable experience for all its visitors," said the Belfast City Council in its statement.

Additional posted footage shows a chimpanzee popping out on to a path used by visitors exploring the zoo.

"I was petrified, obviously, having the kids and I tried not to show fear but inside I was a bit like what happens if it attacks us or tries to take the kids or runs over," Danielle Monaghan, a mother who filmed the incident, told the BBC.

Zookeepers are investigating whether the branch – probably weakened by high winds – fell into the enclosure after they completed routine morning checks, said the city council.

This isn't the first time the Belfast Zoo has had to deal with an escaped animal this year. Last month, the zoo warned nearby residents to look out for a red panda that had escaped. The red panda was later found and returned to the zoo safely.

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