Error: can't open Startup.o

I am using Keil µVision3 & CodeSourcery toolchain for simulating an ARM 
processor.
I created a simple project witch includes main.c file and then added to 
it startup.s from \keil\arm\gnu\startup folder.

But when I build the target, I receive the following message:

compiling main.c...
arm-nl-gnueabi-gcc: main.o: linker input file unused because linking not 
done
assembling Startup.s...
Assembler messages:
Error: Error: can't open Startup.o for reading: No such file or 
directory
Target not created

What is the reason for this problem?