DUBAI (Reuters) - Oman started selling U.S. dollar-denominated multi-tranche bonds on Thursday, a document showed, as the debt-burdened Gulf oil producer seeks fresh funding to cover a large fiscal shortfall. Oman is selling bonds with maturities of 10 and 30 years, and it re-opened $750 million bonds due in 2025, according to a document issued by one of the banks leading the deal and seen by Reuters. Oman, rated below investment grade by all major credit ratings agencies, has piled up debt at breakneck speed over the past few years to offset a decline in oil prices and cover widening deficits. On Thursday, it started marketing the 10-year paper at around 6.625% and the 30-year at 7.625%-7.75%, while the re-opening, or “tap”, of the 2025 bonds was marketed at around 4.875%. The bonds will be of benchmark size, which generally means upwards of $500 million per tranche. Oman is expected to raise $2 to $3 billion with the debt sale, a source familiar with the matter said. The ministry of finance did not immediately respond to a request for comment. Citi, HSBC, JPMorgan and Standard Chartered have been mandated to coordinate the deal, which will be completed later on Thursday, the document said. Oman is also raising funds in the bank market, sources told Reuters earlier this week, working with a group of banks to raise a $1.1 billion facility which could go up to $2 billion in size depending on market appetite. Oman’s external debt maturing this and next year amounts to $10.7 billion, or about 7.5% of gross domestic product, S&P Global Ratings has said. It expects a 2021 budget deficit of 2.24 billion rials ($5.83 billion) this year and to make up the shortfall the government aims to raise about 1.6 billion through borrowing and draw 600 million from its reserves.