This is not much of a warning, but the PG-13 rating (and moreso the UK 12 rating) is somewhat borderline misleading. A little more in the way of the demonic imagery and acts of the creatures, and this could have been a 15, in the UK, and a light R, possibly in the USA. The aliens are ugly in a demonic way, we see a graphic wound shot, there is a very unsubtle drawing of an alien spaceship attacking a home. In reality this almost could've gotten an R "for disturbing images and some terror."
Suggested MPAA rating: rated R for disturbing images, some terror, blood, and some violence
Contains mild to moderate violence, as well as some blood.
There is a very graphic description of a woman's injuries after being hit by a truck and pinned against a tree.
A man tries to see what is in a barricaded pantry by using the blade of a butcher knife; a sharp-nailed "hand" reaches out from under the door and the man chops some of its fingers off. Extremely brief and no blood is visible
A boy nearly dies from a severe asthma attack.
We see a dead dog with a grilling fork in his neck -- a young boy cries over the dog; he apparently killed him after the dog tried to attack his sister. A woman later pulls the bloody fork out.
We hear a dog being killed by something; the dog starts by barking, then whines and whimpers, then becomes silent.
The alien-like fingers grab the neck of a boy through a coal shaft, and extraterrestrial fingers reach under a door.
People are trapped inside a house and we see shadows of something passing by the windows and hear creaking as they walk along the porch; then they begin to pound on the doors and windows trying to get in the house.
A man walks into a dark cornfield, hears noises, drops his flashlight and when he picks it up again it illuminates the leg of a strange creature, and the man runs in terror.
Two men chase something around a house yelling obscenities and threats.
Two men wake up in a start, hear children screaming from a distance, and tear through cornfields to find a little girl and boy standing amidst newly created crop circles.
A dog suddenly snarls and barks at two children, and a dog barks frantically, snarls and jumps at a man.
A man talks about having fallen asleep while driving and hitting and killing a woman walking along the road.
We see an illustration of a house being hit by energy rays from a space ship and three bodies lying in the front yard.
There are continuous reports on TV news about crop circles in fields across the world, sightings of UFOs, the potential for an extraterrestrial invasion, many people having been killed by extraterrestrials using a poison gas, and mass-hysteria.
A video broadcast on TV shows a humanoid creature (a biped) walking in a street as children watch and scream in fear.
A man yells at his children, and a man has a spot of blood on his shirt.