I know that the American Bullfrog (Rana [Lithobates] catesbeiana) is an introduced species in Singapore. The photos are not very clear but it doesn't look like any of the native species. I found a fact sheet on the bullfrog that may help you identify the frog:
Wildlife Singapore: Facts Sheet of American Bull Frog - Rana catesbeiana
Also, a photo essay on the frogs of Singapore:
http://anura-in-singapore.blogspot.com/
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