Okorotchenko was quote as saying that Sri Lanka's short-term commercial debt was a concern. The International Monetary Fund has also warned against short term foreign debt.
"Pakistan is the weakest, followed by Sri Lanka, then Vietnam," Elena Okorotchenko, head of Asian sovereign ratings at S&P, was quoted as saying by Bloomberg, a financial newswire.
"Pakistan faces severe pressure from the external side, the fiscal side, the monetary side, economic growth and politics.
"There are five angles in which we analyze a country's ratings and Pakistan is negative on all counts.''
S&P has downgraded Pakistan to CCC+. The country's foreign reserves plummeted as its central bank tried to defend a dollar peg over more than a year.
Vietnam also had currency troubles as it intervened in the market early in the year without raising interest rates.
Sri Lanka has been defending a dollar peg