* Nickel has gained more than 30 pct this year
* China nickel ore imports from Indonesia down sharply
* Copper slips on worries over Chinese demand
SINGAPORE, April 22 (Reuters) - Nickel futures hit their highest in more than 14 months on Tuesday as an Indonesian ban on mineral ore exports kept supplies tight, with tensions in Ukraine also posing a risk to flows from key producer Russia.
Nickel has gained more than 30 percent this year, outperforming other base metals including copper, and analysts say the rally is unlikely to lose steam anytime soon, with prices well on track to test $20,000 a tonne - a level last seen in early 2012.
China's imports of nickel ore from Indonesia slumped to below 1 million tonnes in March from an average of 3.4 million tonnes a month last year, customs data showed on Monday. Indonesia was the main source of nickel ore for top buyer China before Jakarta banned shipments of unprocessed minerals from January.
Prices for nickel ore, often used as a substitute for refined nickel by stainless steel plants, have surged in China, raising prospects that mills will use refined nickel instead of the ore.
"With the Indonesian ban there's really no other good source for nickel ore and that's freaking out the market," said Dominic Schnider, analyst at UBS Wealth Management.
Three-month nickel on the London Metal Exchange hit a session high of $18,275 a tonne, its loftiest since Feb. 18 last year. It was up 1.8 percent at $18,255 by 0711 GMT. The LME reopened on Tuesday after a four-day Easter holiday weekend.
Nickel has fallen only on a single trading day this month.
Risks that the conflict in Ukraine may disrupt supplies from Russia also supported nickel prices. Russia's Norilsk Nickel is the world's largest nickel producer.
U.S. and European officials say they will hold Russia responsible and impose new economic sanctions if pro-Moscow separatists do not clear out of government buildings they have occupied across eastern Ukraine over the past two weeks.
"Clearly nickel is getting support from two angles - one is more of a tail risk that hasn't disrupted flows, while the other one has," said Schnider.
"I think the market is going to chase it hard, $20,000 is a possibility."
While nickel raced higher, London copper fell 0.6 percent to $6,611.75 a tonne, still pressured by worries over Chinese demand amid a slowdown in the world's top user of the industrial metal.
The most-traded July copper contract on the Shanghai Futures Exchange eased 0.2 percent to close at 46,430 yuan ($7,500) a tonne.
HSBC will release its China flash Purchasing Managers' Index for April on Wednesday. The HSBC PMI has been below 50 since January, suggesting small Chinese manufacturers have been struggling this year amid slower economic growth.
China's economy grew 7.4 percent in the first quarter, the slowest pace in six quarters, according to government data released last week.
Three month LME copper
Most active ShFE copper
Three month LME aluminium
Most active ShFE aluminium
Three month LME zinc
Most active ShFE zinc
Three month LME lead
Most active ShFE lead
Three month LME nickel
Three month LME tin
($1 6.2274 Chinese yuan)
(Reporting by Manolo Serapio Jr.; Editing by Joseph Radford and Subhranshu Sahu)