Simply: No they're not
With ab exercises you train your muscles, the body takes it's energy from every source it got, no matter where it's needed. So you can't specifically burn fat at a particular bodypart.
But as you want to reduce your belly fat, why not also define some nice muscles beneath it...
It's absolutely true that you cannot burn fat at a particular body part - in other words, you can't target fat reduction or "spot reduce" fat on any part of your body. However, the idea that abs exercises do not burn fat is a popular misconception. It comes about from misunderstanding the principles behind fat burning and muscle building. When developing a nice set of six pack abs, there are two main parts: 1) fat burning in order to "uncover" the abdominal muscles; 2) muscle building in order to "build" the abdominal muscles and make them appear more "chiselled" and bumpy, which is really just making them slightly bigger and stronger. I emphasise slightly because if you have overdeveloped abs, then it will give you a somewhat bloated look (trust me, you don't want this - a lot of bodybuilders have it and it just looks exaggerated).
To answer your original question, ab exercises are not necessarily more useful for reducing belly fat than other exercises, but they are still VERY useful. To clarify what I mean by this, here are a couple of questions to think about:
- Do ab exercises use up energy and thus burn calories?
- Yes - Of course they do; they put tension on the abdominal muscles
- Do ab exercises temporarily break down the abdominal muscles resulting in strengthening of those muscles in between training sessions?
- Yes - Any exercise which isolates or targets a single muscle group is technically a strength building exercise
- Do both of the above "burn fat" so to speak?
- Yes - burning calories creates a calorie deficit which, eventually, results in a fat loss (including around your stomach!); strengthening abdominal (or any other) muscles also burns additional calories at rest by increasing your metabolism.
So the benefits of ab exercises are:
- they strengthen you abdominal muscles, which will produce that chiseled look once you've stripped away all the remaining fat
- they DO assist in fat burning because, like strengthening ANY other muscle, strengthening your abs boosts your metabolic rate at rest
What does this mean for your training regime? It means that if you were to try and sculpt out your abs by doing ONLY abs exercises, then it would probably take you a lot longer, simply because you would not be taking advantage of all the other ways of burning fat such as interval training and steady state cardio. It also means, however, that if you're already doing regular cardio and you intend to continue doing it, then you can also increase your volume of abs exercises in addition to whatever other training your doing.
I have recently increased the volume or amount of abs exercises I'm doing because I already do a lot of cardio (such as walking) and strength training, so for me personally, there doesn't seem to be much room for increasing what I'm doing in other areas besides abs work. The other main alternative to getting past a fat burning plateau is to increase the weight you are lifting in your strength training exercises, since that will then stimulate your other muscle groups to grow more muscle which will consume more calories at rest. And a nice side effect of doing that, of course, is that you'll also become more ripped and developed in the rest of your body, not just your abs.
My only word of caution with doing abs exercises is to be careful about your form (or technique). There's plenty of videos on youtube which demonstrate correct form so as to avoid a back injury.