Including header file in assembly file

I am trying to include a header file containing a macro into my main assembly file, but the compilation fails.

Below is my main.S file

#include "common.h"
BEGIN
    mov $0x0E40, %ax
    int $0x10
    hlt

Below is my common.h file :

.macro BEGIN
    LOCAL after_locals
    .code16
    cli
    ljmp $0, $1f
    1:
    xor %ax, %ax
    /* We must zero %ds for any data access. */
    mov %ax, %ds
    mov %ax, %es
    mov %ax, %fs
    mov %ax, %gs
    mov %ax, %bp
    /* Automatically disables interrupts until the end of the next instruction. */
    mov %ax, %ss
    /* We should set SP because BIOS calls may depend on that. TODO confirm. */
    mov %bp, %sp
    /* Store the initial dl to load stage 2 later on. */
    mov %dl, initial_dl
    jmp after_locals
    initial_dl: .byte 0
after_locals:
.endm

Both files are in same directory. When I do the compilation :

$ as --32 -o main.o main.S
main.S: Assembler messages:
main.S:2: Error: no such instruction: `begin'

What am I missing? I did a little research and got this answer in SO, but its not helpful. Please help.