I want to lose belly fat but I'm already underweight.

Well you should try to put on some weight but there is no harm in doing some stomach exercises at the same time, to help tone your belly

Here is a link to some really good pilates exercises that focus on the belly, www.inch-aweigh.com/pilates_exercises.html

Good luck!

Strength training and weight lose are diametrically opposed. Do the strength training first, then work on fat loss if that proves necessary.

JayR

If your already underweight and your having a big belly, that could be another health problem.  Are you thin and look like underweight?  First seek for medical attention and have your body check.  But you may also choose types of fitness training that targets to flatten your belly.

You can do some exercising for the stomach muscles.  I wouldn't go on a diet though because as you say you are already under weight.  Try the exercising first to strengthen the abs.

It is alright to lose your belly fats even you are underweight. To lose your belly fats, try half sit-ups for 10-15 minutes per day. I will guarantee you that you will lose your belly fats less than 2 to 3 weeks.

try a combination of weight training and intervals. Intervals are very effective in burning fat. try to look for a workout called turbulence training, also, check out this site for maximum fat loss

Tummy exercises will help you lose that tummy.

San Diego Personal trainer

As you are underweight, so I would like to suggest you to gain the right weight according to your height. Then for losing belly fat you can choose some exercises that are effective for that area. Strength training is certainly an option. Be healthy. Good luck.

Source: http://www.complete-strength-training.com/strength-training-for-women.html