Right at the base? Cutworms.

Stick about 2-3 toothpicks right next to the stalk so they can't wrap around it and snip them. Never fails.

Kevin

How can I kill these worms?

Find them and squish them. Bt K variant may kill them but probably not before they nip the plant. Diatomaceous Earth supposedly controls them but is ineffective when wet and will kill beneficial insects, and you'll probably still lose the seedlings. Beneficial nematodes will work, but they're expensive and take time.

I gave you a 100% cure(since I started using them)to prevent them from nipping your seedlings. No toothpicks? Use twigs. Use nails.

Kevin