South Korea's new President Moon Jae-in backed proposals for a collaboration after it was suggested by sports minister Do Jong-hwan.
But North Korean International Olympic Committee (IOC) member Chang Un has dismissed the idea - saying there was not time to negotiate a deal.
The Games in Pyeongchang, South Korea, will take place from 9-25 February.
The two sides have played in the same team before - at the 1991 World Table Tennis Championships.
However, Mr Chang told local media: "It took us 22 rounds of talks to set up that joint [table tennis] team... it took us five months.
"That's the reality we face."
South Korea's sports minister had suggested a joint ice hockey team - even going as far as to suggest they might allow the north to host skiing events - to help make the 2018 games a "peace Olympics".
President Moon, who advocates greater dialogue with South Korea's neighbours, then put forward the idea of a wider unified Olympic squad.
But Mr Chang said the games should not be used for political purposes, adding: "As an expert of the Olympics, it is a little late to be talking about co-hosting. It's easy to talk about co-hosting, but it is never easy to solve practical problems for that. It's the same for forming a joint team for ice hockey."
South Korean officials have said they continue to be open to the idea.
The two sides remain technically at war as the fighting at the end of the Korean War in 1953 did not end with a peace treaty. Tensions have risen recently following repeated missiles tests carried out by Pyongyang.

Aaron McEneff put the Candystripes in control with two first-half penalties, both given for fouls on Lukas Schubert.
Nathan Boyle fired home early in the second half and Schubert hammered in the fourth before Keith Ward's late lob completed the Finn Park rout.
Derry go a point ahead of Cork City and the teams meet at the Brandywell on Monday night in a crunch encounter.
It was Derry's first win over their north-west rivals in three meetings this season and they were rarely troubled by a disappointing Harps side.
McEneff tucked away the opener on 17 minutes after Ciaran Coll's challenge on Schubert, although the foul appeared to be outside the box.
There was no doubt the second spot-kick, with former Candystripes midfielder Barry Molloy bringing downing Schubert and McEneff making it 2-0 three minutes from the break.
Boyle produced a clinical finish from a Dean Jarvis cut-back eight minutes after the restart.
City keeper Gerard Doherty had a quiet night but he made a fine save to deny David Scully in the host's best opening of the game.
Schubert flicked the ball past Coll before blasting in after 83 minutes and it was soon followed by Ward's clever chip over Harps stopper Richard Brush.
It was Derry's biggest win of the campaign and perfect preparation for the showdown with Cork.

Investors Steve Kaplin and Jason Levien, who run sports teams in the United States, are drafting in the former Everton striker as an adviser.
The £100m takeover was agreed subject to Kaplin and Levien passing a Premier League test for prospective owners.
But Donovan, now 34, will not take part in Swansea's day-to-day activities.
Current chairman Huw Jenkins and deputy Leigh Dineen will retain their roles when the Americans take control but Donovan is seen as ideal to have on the board with football and commercial expertise.
Levien, the managing general partner of Major League Soccer side DC United, and Kaplan, head of a capital investment fund and vice-chairman of NBA franchise Memphis Grizzlies, lead a consortium which has bought a controlling stake of 60% in the club.
Donovan, who was on loan at Premier League side Everton in 2010 and 2012, is his country's record goalscorer and the second most capped United States international.
The former LA Galaxy and San Jose Earthquakes forward also had spells in the Germany Bundesliga with Bayer Leverkusen and Bayern Munich.
North Korea has rejected an offer from the South to form a unified team for next year's Winter Olympics.
[NEXT_CONCEPT]
Derry City have moved up to second in the Premier Division thanks to a thumping 5-0 derby win over Finn Harps.
[NEXT_CONCEPT]
Former United States captain Landon Donovan is to be part of the American investment group taking over Premier League club Swansea City.