My newborn kittens are dying one by one anyone help?

I rescued a pregnant cat and she has given birth 2 days ago to 4 babies. They all seemed to be healthy and strong and feeding well. Mother then began moving three of them away but leaving one. This baby died yesterday. Again last night another baby died. Mother is reluctant to stay with them now. The remaining two are weak looking and making a shrill screaming sound and writhing as if in pain. My vet's not in surgery for a few more hrs and i am unable to reach him. Why are these kittens gagging as if for air? I fear they are all going to die. I've placed the babies near mother's but they aren't feeding. Mother has made her available but they aren't feeding, just clawing and writhing on her so she leaves them. Please anyone help?