At around 9pm Saturday, police received a report from a Singapore Airlines employee that four kittens were found on a cargo plane operated by Singapore Airlines, which arrived in Hong Kong hours earlier.

The two black and two white kittens were found in the cabin.

Police arrived and notified the Agriculture, Fisheries and Conservation Department for follow up.

As the AFCD has killed animals that have not been quarantined in the past, it caused concerns among animal protection groups.

Volunteers launched a petition campaign on social media, urging the department to spare their lives.

"Although the four kittens have not been quarantined, Singapore is not a an area affected by rabies. The AFCD can give the four cats to animal protection groups to arrange for adoptions after quarantine," volunteers said.

The department said initial examinations found that the four cats were in good health and a veterinary check up would be arranged later.

They are now under quarantine and taken care of by AFCD staff, in accordance with the rabies risk assessment requirements.

"Upon completion of the quarantine and investigation, adoption will be arranged through partnering animal welfare organizations according to the condition of the cats," AFCD said.