(Recasts throughout, add details, rumoured terms)
LONDON/KIEV, Sept 14 (Reuters) - Ukraine's state energy firm Naftogaz is set to begin talks with some creditors this week as part of a $1.6 billion debt workout, sparking intense speculation about restructuring terms of a $500 million Eurobond maturing this month.
Neither Naftogaz nor its appointed advisor for the restructuring, Credit Suisse, would reveal the likely details of the proposal. But analysts said it was likely to include an extension of the tenor by another five years as well as an explicit government guarantee on the bonds.
Talk of a 9.5 percent coupon on the newly issued notes was also rife.
In a brief statement, Naftogaz -- at the centre of energy rows with Russia in recent years -- said it was working to formalise the details of its restructuring proposal.
"Naftogaz is aware of the impending maturity of the Eurobond and remains focused on providing a clear and robust proposition to bondholders as soon as practicable," Naftogaz said.
The company unsettled investors last month when it said it
wanted to restructure its external debt, including a Eurobond
A forced restructuring or default on Sept. 30 could hit all Ukrainian bonds -- state and corporate -- as many investors see the Naftogaz debt as quasi-sovereign.
Citing local sources, Commerzbank analyst Luis Costa said the restructuring was unlikely to come with an upfront cash repayment but he said a new coupon of 9.5 percent was likely to replace the current 8.125 percent.
"Current valuation on Naftogaz '09 bonds suggests a 140-150 bp (basis points) spread over sovereign in the new restructured bonds, which sounds like a pretty tight (expensive) valuation," Costa wrote in a note.
Naftogaz has required constant government support as it buys increasingly expensive gas from Russia but then sells it on a subsidised, and often lower, prices domestically.
The government has promised to shore up the finances of Naftogaz, starting with raising household gas and utility prices, as part of its $16.4 billion programme with the International Monetary Fund.
"The decision to restructure is clearly political and the way it's been handled lacks transparency. But the risk of a substantial loss for Naftogaz bondholders seems to be modest," said Douglas Busvine, an analyst covering Russia and the CIS for Medley Global Advisors in Vienna. (Reporting by Sebastian Tong and Mike Dolan in London and Sabina Zawadzki in Kiev; Editing by Ron Askew)