The amount of trichlorofluoromethane (CFC-11) in the atmosphere has been sinking more slowly since 2012 than should be expected, scientists at the National Oceanic and Atmospheric Administration in Colorado wrote.
CFC-11 still contributes about a quarter of all chlorine - the chemical that triggers the breakdown of ozone - reaching the stratosphere.
Montzka told the BBC that the data points "fairly definitively" towards Eastern Asia, somewhere around China, Mongolia and the Koreas.
But they concluded these sources could not explain the increase, which they calculated at about 13 billion grams per year in recent years. Though production of CFCs was phased out by the Montreal Protocol, a large reservoir of CFC-11 exists today primarily contained in foam insulation in buildings, and appliances manufactured before the mid-1990s.
Last fall, it was reported that the hole in the Earth's ozone layer had shrunk to its smallest size since 1988, which was great news.
An ozone depleting CFC refrigerant, thought to be virtually extinct following Montreal Protocol phase outs, has mysteriously reappeared in increasing amounts in the atmosphere.
The slowdown in reduction of CFC-11 also has implications for the fight against climate change. "Further work is needed to figure out exactly why emissions of CFC-11 are increasing and if something can be done about it soon".
Although Montzka and his colleagues could not pinpoint the exact location of the new emissions, some of their observations and models offer clues as to where they might be coming from.
"I hope that somehow the worldwide community can put pressure on South East Asian countries, maybe China, to go and look at whether they can get more information on where the emissions come from".
Two years after the discovery of the hole in the ozone layer above the Antarctic in 1985, the Montreal Protocol was signed, an global treaty which introduced restrictions on the production of CFCs.
Keith Weller, a spokesman for the United Nations Environment Program, which administers the Montreal Protocol, said the findings will have to be verified by the scientific panel to the Protocol, and then would be put before the treaty's member countries.
The UNEP said that is was "critical that we take stock of this science, identify the causes of these emissions and take necessary action".
Unreported production of CFC-11 outside of certain specific carve-out purposes in the treaty would be a "violation of global law", Weller confirmed, though he said that the Protocol is "non-punitive" and the remedy would probably involve a negotiation with the offending party, or country.
"They're going to find the culprits". In 2012, however, the rate of decline suddenly reduced by about 50% - indicating that new source of production had started up. If not remedied soon, however, substantial delays in ozone layer recovery could be expected, Montzka said.
"Knowing how much time and effort and resources have gone into healing the ozone layer, and to see this is a shocker, frankly", said Montzka.