Probably Roundworms, what I have used in the past is Drontal Plus which is effective against roundworms, hookworms, whipworms and tapeworms. This basically to me is a good 1st wormer of course ask your Vet about it first. I used this on my rescue Dobe, I wanted to worm him for as much as many parasites as possible.
Don't forget to follow up with a wormer such as Strongid T in 2 weeks to kill off the eggs that mature in your pup. I chose Strongid T as it is economical and will do for the roundworms which can reinfest your dog. Drontal plus first (it cost a lot for Drontal Plus)
Make sure in the meantime that if you are handling your pup to wash your hands throughly. The eggs which are unseen to the naked eye can be ingested easiely and will stick to the rear end of the dog. Roundworms can be very dangerous in humans as they have a different migration tour of the body. The disease in humans that dog round worms can cause is called visceral larva migrans.
Also keep in mind to keep your yard clean and pooper scooped ASAP. Round worm eggs can lay dormant in the soil for years...also your dog could reinfest itself. One study of parks in a large city in the U.S revealed that one of three soil samples contained T.canis eggs...scary.
Currently I give my dogs a natural wormer, but, in your case I would probably go to the Vet and make sure you get a wormer to get rid of them ASAP.