* PM says "extremely difficult" period ahead

* Says continued co-operation with the IMF necessary

* But finance minister says no risks to economy, currency

(Adds Tymoshenko, details)

By Natalya Zinets

KIEV, Nov 11 (Reuters) - Ukraine's economy will undergo an "extremely difficult" period without a $3.8 billion bailout from the International Monetary Fund, Prime Minister Yulia Tymoshenko said on Wednesday.

Her comments mirror those of Deputy Prime Minister Hryhory Nemyrya who said payment for Russian gas will become increasingly difficult without the IMF cash -- part of a $16.4 billion programme to counter economic crisis.

The IMF refrained from releasing the funds after parliament passed a bill raising the minimum wage by over 20 percent. President Viktor Yushchenko -- bitter rival to Tymoshenko -- brought the law into force by approving it last month.

This has sparked accusations from Tymoshenko that Yushchenko deliberately sabotaged the IMF deal ahead of a presidential election in January in which both will run. With public support of 5 percent or less, Yushchenko is unlikely to win re-election.