In recent weeks, deepening trouble in the U.S. subprime-mortgage market has spilled over into global credit and equity markets, causing turmoil and prompting some investors to slash their exposure to risky assets, including those in emerging markets.

S&P's so-called Liquidity Vulnerability Index aims to measure the sovereign vulnerability of emerging countries to negative market conditions. The five most vulnerable countries in the recent survey were Latvia, Iceland, Bulgaria, Turkey and Romania, S&P said in a report released Monday.

In contrast, the four countries least at risk are Russia, Egypt, Ukraine and the Czech Republic. The other sovereign lenders surveyed were South Africa, Hungary, Poland, Slovakia, Lebanon and Lithuania.

"We have not, to date, changed any ratings or outlooks due to the tighter financial conditions," S&P credit analyst Moritz Kraemer said in a statement. "Over the past 24 months, however, we have lowered ratings or outlooks for a number of sovereigns in the sample with deteriorating credit fundamentals."

Countries that rank most vulnerable on the Liquidity Vulnerability Index have suffered the most negative ratings and outlook changes in recent months, but placing a country in the vulnerable group doesn't necessarily predict negative rating action, S&P said.

"Everything depends on the policy reaction by the authorities," Kraemer said. "Indeed, Standard & Poor's last week affirmed the ratings and stable outlook on Turkey, despite the country's well above-average vulnerability to changes in market sentiment."

"The affirmation reflects our expectation that the government's policy mix will continue to mitigate Turkey's refinancing risks," he said.

S&P's Vulnerability Index is made up of several measures, including sovereign debt rollover needs as a share of GDP, external borrowing requirements as a share of current-account receipts and the share of the current-account deficit financed through foreign direct investment.