BANGKOK, Aug 11 (Reuters) - Thailand will sell 21 billion baht ($594.40 million) worth of government savings bonds later this month to help finance a budget deficit, a finance ministry official said on Thursday.

The 5-year and 10-year bonds will be offered to the public between Aug. 22 and 24, with a yearly average coupon of 2.9% and 3.6%, respectively, said Patricia Mongkhonvanit, head of the ministry's public debt management office.

The country's public debt to GDP ratio is expected at 61.3% at the end of the current fiscal year to September, down from a previous forecast of 62.69%, she told a briefing. The approved debt ceiling is 70% of GDP.

Some state agencies have not borrowed as previously proposed, she added.

As rising interest rates will affect public borrowing costs, the government has tried to adjust its existing debt to fixed interest rates and turned to longer-term debt, she added.

In the 2023 fiscal year starting in October, the amount of government bonds to be issued should not be very different from the current fiscal year, he added.

About 1.04 trillion baht ($29.46 billion) of government bond issues are planned for the current year, according to the ministry

($1 = 35.33 baht) (Reporting by Kitiphong Thaichareon Writing by Orathai Sriring; Editing by Kanupriya Kapoor)