A Chinese fertilizer maker facing a bond repayment next month is among four borrowers flagged as the nation's riskiest by Haitong Securities Co after the first State-firm default.

Inner Mongolia Nailun Group Inc must pay 540 million yuan ($87 million) in principal and interest on an 8 percent note on May 20, according to lead underwriter Chinalin Securities. Packaging maker Zhuhai Zhongfu Enterprise Co's 590 million yuan of 5.28 percent securities due May 28 was also on Haitong's list in a report dated Wednesday.

Chinese companies are struggling with an economic slowdown as a report on Thursday showed a manufacturing gauge fell to a 12-month low in April. Borrowing costs for lower-rated companies have risen after Baoding Tianwei Group Co, a power-equipment maker, became China's first State-owned enterprise to default on domestic debt earlier this week.

"China's default risks are rising as the economic slowdown is crimping corporate profits," said Li Ning, a bond analyst in Shanghai at Haitong, the nation's third-biggest brokerage. "We may see more defaults in the onshore market following Baoding Tianwei."

An official at the office of Zhuhai Zhongfu's board secretary, who asked not to be identified, said the company will be able to repay the debt due next month.

Haitong also flagged China Erzhong Group Deyang Heavy Industries Co's bonds due in October and paper maker Henan Yinge Industrial Investment Holding Co's 2017 debentures that investors are able to sell back in December.

The yield premium on 10-year AA-rated corporate bonds over sovereign notes widened two basis points on Wednesday to 329 bps. The gap climbed five bps the previous day, the biggest increase in four months, as Baoding Tianwei defaulted.

Inner Mongolia Nailun owes overdue interest on a loan, Pengyuan Credit Rating Co said in a Feb 9 report. It also has an interest payment due on a separate note on May 5, Chinalin Securities said on March 12.