Experts believe they are just two years away from creating a hybrid embryo which would mark a turning point in plans to haul the beast back to life by programming its key traits into an Asian elephant.
The bundle of cells would have genes for mammoth features such as shaggy long hair, thick layers of fat and cold-adapted blood.
However, years more work lie ahead before any serious attempt can be made to produce a living creature.
The US scientists have ambitious plans to grow it within an artificial womb rather than recruit a female elephant as a surrogate mother.
Since starting the project in 2015 the researchers have increased the number of “edits” where mammoth DNA has been spliced into the elephant genome from 15 to 45. Professor George Church, of Harvard University, said: “Our aim is to produce a hybrid elephant/mammoth embryo. Actually, it would be more like an elephant with a number of mammoth traits.
“We’re not there yet, but it could happen in a couple of years.”
The woolly mammoth roamed across Europe, Asia, Africa and North America during the last Ice Age and vanished, probably owing to climate change and hunting. Their closest living relative is the Asian, not the African, elephant.
“De-extincting” the mammoth has become a prospect because of revolutionary gene editing techniques that allow the precise selection and insertion of DNA from specimens frozen over millennia in Siberian ice.
Prof Church said the mammoth project had two goals – securing an alternative future for the endangered Asian elephant and helping to combat global warming.
Woolly mammoths could help prevent tundra permafrost from melting and releasing huge amounts of greenhouse gas into the atmosphere.
They keep the tundra from thawing by punching through snow and allowing cold air to come in,” said Prof Church. “In the summer they knock down trees and help the grass grow.”
Research shows mammoths in Siberia could cause local temperatures to drop by up to 20C. Gene editing is a topic under discussion at the American Association for the Advancement of Science meeting in Boston, Massachusetts.