The pair have been included in the England squad on merit and not in an attempt to lock down their international futures, Southgate has said.
Abraham, whose father is Nigerian, has never been capped in a competitive game for England but has made two friendly appearances for the Three Lions.
Abraham's Chelsea teammate, Tomori, was born in Canada and is also eligible for Nigeria through his parents.
"I don't think it's ethically right just to give a player a cap to make sure they can't play for somebody else," said Southgate. "So, the two boys that are in the squad this time, Fik [Fikayo Tamori] and Tammy, are in on merit. I'm not going to guarantee that they're going to get on the pitch, but they've both been part of our junior system right the way through.
"Fik came in a little bit later than Tammy but has been part of the youth team that won the World Cup and as far as we're concerned, born in England, Tammy, the two boys have been developed through English football, they're English players and I'm sure they have huge desire to play for England at senior level."