(Adds details on loan; rating downgrade)

KIEV, Nov 12 (Reuters) - Ukraine's state railway company is seeking to restructure a $550 million syndicated loan organised by Barclays (BARC.L: Quote, Profile, Research) after failing to repay a portion of it, acting Finance Minister Ihor Umansky said on Thursday.

Ukrzalyznitsya's attempt to change the terms of the loan comes hot on the heels of a restructuring by the state energy company Naftogaz of its entire foreign debt through an issue of a $1.65 billion bond, and after the IMF refrained from releasing the latest instalment of a bailout package for Ukraine.

The railway company "should have paid $110 million of the debt and $8 million in coupon payments," Umansky told reporters. "(The company) has made some proposals to the holders of the debt, which was organised by Barclays."

Fitch Ratings downgraded Ukraine's sovereign credit rating on Thursday to 'B-' from 'B' with a negative outlook.

The railway company's syndicated loan of $550 million was organised in July 2007 and matures next year, according to Thomson Reuters Loan Pricing Corp data. It had a 2-year grace period, which would have run out this summer.

Umansky said the size of the loan was $440 million after the company made some of the principal payments. Ukrzalyznitsya itself was not immediately available for comment.

Prime Minister Yulia Tymoshenko said on Wednesday Ukraine's economy would undergo an "extremely difficult" period without a $3.8 billion payment from the International Monetary Fund, 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. [ID:nLB138580]