(Reuters) - The International Monetary Fund's bailout program for Ukraine assumes Kiev will be able to get $15.4 billion from talks with its creditors, according to four sources familiar with the IMF's documents.

The assumption is necessary to ensure Ukraine's sovereign debt can fall to 70 percent of gross domestic product by 2020, a level the IMF would deem sustainable, according to three people.

Under its rules, the IMF cannot lend to countries unless it believes they will be able to pay back the money eventually.

Targeting a particular level for debt renegotiation, considering debt talks have not yet begun, points to the uncertainty surrounding the $40 billion international rescue package for Ukraine announced last month.

After a year of political upheaval and war, Ukraine's economy is in tailspin with a currency that just pulled back from record lows and the highest interest rates in 15 years.

Under the IMF program, Kiev must make deep changes to its energy sector and banking system, and tackle decades of corruption, even as it battles pro-Russia separatists in its eastern regions.

An IMF spokeswoman was not immediately available to comment. Last month, the IMF declined to share details of the financing package, and said it had not made any assumptions about a debt restructuring for private creditors.

The Washington-based fund announced a preliminary agreement for a new $17.5 billion, four-year loan program for Ukraine last month, alongside pledges from the World Bank, the United States, the European Union and other countries.

The IMF's board is widely expected to approve the program when it meets on Wednesday after Kiev passed a new draft budget last week to help it clinch the deal.

Finance Minister Natalia Yaresko also said the Ukrainian government expects to get $15 billion from talks with holders of its sovereign bonds, and should begin the discussions after the IMF deal goes through.

Boutique investment firm Rothschild has invited investors to organize a Ukraine creditor group ahead of the talks, which are expected to be difficult in light of Ukraine's ongoing war, sources familiar with the situation told Reuters.