The don't specify what and where the terrorists are from, but it is know that it all takes place in the south parts of Russia, near arab countries, so they are basicly the (nowadays) classic muslim terrorists. not specificly russian terrorsts, more like multi national terrorists.
And for those who don't like to be a US soldier, you play most of the game a a Brit
BTW I don't see the problem with the whole, OMG I have to fight the russians..
I mean common Russia is still our most loved enemy.
Countless movies are made with the russians as the bad guys, but it all lost it's glamour after the iron curtain fell, after that the commie treat was gone so they had to find a new enemy to use, and first it was China (and sometimes it still is (recent James Bond movies (exept Casino Royal), Battlefield 2 you name it) but now it is all Terrorists shooting etc.
Look I have nothing against the Russians, or any other nation, let that be clear.
Man I love WWII shooters and I was even born in Germany and you don't see me complaining that you have to shoot germans, and in campain always have to play as the good guys