GENEVA, March 16 (Reuters) - Nine out of 10 Ukrainians could face poverty and extreme economic vulnerability if the war drags on over the next year, wiping out two decades of economic gains, the U.N. Development Programme (UNDP) said on Wednesday.

Achim Steiner, UNDP Administrator, said that his agency was working with the Kyiv government to avoid a worst case scenario of the economy collapsing. It aimed to provide cash transfers to families to buy food to survive and keep them from fleeing while propping up basic services.

"If the conflict is a protracted one, if it were to continue, we are going to see poverty rates escalate very significantly," Steiner told Reuters.

"Clearly the extreme end of the scenario is an implosion of the economy as a whole. And that could ultimately lead to up to 90% of people either being below the poverty line or being at high risk of (poverty)," he said in a video interview from New York.

The poverty line is generally defined as purchasing power of $5.50 to $13 per person per day, he added in a video interview from New York. Before Russia launched its invasion on Feb. 24, an estimated 2% of Ukrainians lived below the $5.50 line, he said.

Ukraine's top government economic adviser Oleg Ustenko said last Thursday that invading Russian forces have so far destroyed at least $100 billion worth of infrastructure and that 50% of Ukrainian businesses had shut down completely.

"We estimate that up to 18 years of development gains of Ukraine could be simply be wiped out in a matter of 12 to 18 months," Steiner said.