More US companies have defaulted on their debt this year than issuers from any other country or region, S&P analysts led by Diane Vazza wrote in a December 24 report.

As of last week, 111 companies worldwide had defaulted on their obligations, the highest tally since 2009 when the the figure hit 242 for the same period.

About 60 per cent of this year's global defaults have come from US borrowers, Vazza wrote, up from 55 per cent a year ago, when 33 of 60 defaulters were American.

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After the United States, companies from emerging markets were the second-largest defaulters, accounting for 23 per cent of the pool, which is a smaller share than last year, according to S&P data.

Plummeting oil prices and speculation about how the Federal Reserve's plan to tighten monetary policy would affect corporate borrowing costs has made companies more vulnerable, Vazza wrote.

"The current crop of US speculative-grade issuers appears fragile, and particularly susceptible to any sudden, or unanticipated shock," she wrote.

Arch Coal was the most recent addition to the list, having its credit rating downgraded to "speculative default" by Standard & Poor's last week after the coal producer missed about US$90 million in interest payments and exercised a 30-day grace period with the holders of some of its notes.

Looking ahead, S&P expects the US corporate default rate will rise to 3.3 per cent by September 2016 from 2.5 per cent a year earlier. The bulk of the failures will come from companies in the oil and gas sector, which accounted for about a quarter of this year's defaults.