I want to implement a fps bot for my Artificial Intelligence project. What would be the best engine/framework to create this in since i want to concentrate on the AI code only without worrying about other details like the graphics and animation. I have looked at Unity and UDK as possible candidates. The main question is that are there art assets available for these engines that I can directly use in my project like maps and characters.

Have you considered writing a bot as a mod for a game, instead of from a kit like Unity/UDK?
e.g. if you mod an Unreal Engine game that has a modding SDK, then it will likely similar to the UDK (seeing the UDK is just a free Unreal Engine), but with a FPS game's assets and game rules ready to go.

I have considered modding using the Source SDK as well but i wanted to use this project as a learning experience for an engine like UDK. I am also considering open source alternatives like Panda3D. As i have said before the main constraint is the availabiity of the resources needed for this project.

I also would recommend using some existing game that allows for creating bots. The distractions of dealing with exactly the things you mentioned can be overwhelming and force you to learn/hone skills that aren't directly related to what you are trying to accomplish (AI).