Story highlights Victims appeared to have been trapped by grills on windows

Minister: 29 similar incidents involving fires at religious schools.

Kuala Lumpur (CNN) At least 21 students have been killed after a fire broke out at a school in the Malaysian capital of Kuala Lumpur early Thursday morning.

Eyewitnesses reported being woken by cries for help from inside the burning building, where children appeared trapped by metal window grills.

"I saw children kicking on the grill but they couldn't get out. My friends and I rushed over and tried to reach them but we couldn't get in," eyewitness Shahirman Shahril told CNN.

When emergency responders first arrived on the scene, "almost 90% of the building was already on fire," a Fire and Rescue department official told reporters.

Two adults also died in the the blaze, while four other victims suffered serious injuries.

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