osdev linker "file format not recognized"

I'm trying to make a simple OS, so I wrote a small kernel that consist out of a c file an assembly file that calls the main function in the c file.
but when I try to compile and link the two files together I get a file format not recognized error by the linker.

this is my Makefile:

BUILD?=build/
ASM?=nasm
ASMFLAGS?=-f obj
CC=i386-elf-gcc
LD=i386-elf-ld
CCFLAGS=-m32 -std=c11 -O2 -g -Wall -Wextra -Wpedantic -Wstrict-aliasing
CCFLAGS+=-Wno-pointer-arith -Wno-unused-parameter
CCFLAGS+=-nostdlib -nostdinc -ffreestanding -fno-pie -fno-stack-protector
CCFLAGS+=-fno-builtin-function -fno-builtin
LDFLAGS=

SOURCES_C=$(wildcard *.c)
SOURCES_ASM=$(wildcard *.asm)
OBJECTS_C=$(patsubst %.c,$(BUILD)/kernel/c/%.obj,$(SOURCES_C))
OBJECTS_ASM=$(patsubst %.asm,$(BUILD)/kernel/asm/%.obj,$(SOURCES_ASM))

all: kernel.bin

kernel.bin: $(OBJECTS_ASM) $(OBJECTS_C)
    $(LD) -o $(BUILD)/kernel.bin $< $(LDFLAGS) -Tlink.ld    

$(BUILD)/kernel/c/%.obj: %.c always
    $(CC) $(CCFLAGS) -o $@ $<

$(BUILD)/kernel/asm/%.obj: %.asm always
    $(ASM) $(ASMFLAGS) -o $@ $<

always:
    mkdir -p $(BUILD)/kernel/c
    mkdir -p $(BUILD)/kernel/asm

I also tried to change the -f flag in nasm to elf, but the I get a undefined reference to main error.

Here is my assembly file calling main:

[bits    32]
[extern main]

section .text
global _start
_start:
    call main

And my C file with the main function:

void main(){
    volatile char* video_memory = (volatile char*)0xb800;
    *video_memory = 'X';
}

So does anyone have an idea on what is causing the problem?