Pain in left hand & wrist with barre chords?

I`ve recently started learning barre chords. I find that "A shape" (is that what you`d call them?) barre chords (with the root on the 5th string) cause pain in the back of my left hand and wrist after about 15-20 minutes of using them. I try to keep my wrist as straight as possible, and I try to apply some pressure with my thumb, but I`m still getting pain and fatigue. Is this normal at first? Will it go away once I`ve developed strength in my left hand, or is it a warning sign of impending RSI? Thanks

You`re exercising new muscles, so yes it will hurt while you`re building up strength, and yes it will get easier to hold these shapes for longer. Having said that, I`m not medically qualified, so if in any doubt take a rest.
And while you`re resting, give yourself a pat on the back (with your other hand ...), as "A" shape barre chords are tricky to master.

Thanks for the reassuring reply. I guess I`ll just keep plugging away, unless the pain gets really severe.

guitar down around his crotch area because it looks "punky" (and, incidentally, he is a punk... A 22 year old punk, in fact, but that`s another story). I mostly practise while sitting in an armless chair with my guitar resting on my left leg. When I stand up, I have the back of the guitar at my belly-button level. Yes, it looks pretty goofy by today`s standards, but I`m not too concerned.