Although there were "no or minimal losses to creditors", the delay suggests institutional weaknesses and coordination gaps within Vietnam’s administration "pointing to creditworthiness that may no longer be consistent with a Ba3 rating", Moody’s said in a statement on Wednesday.

These weaknesses seem to reflect deficient coordination and planning among various arms of the government, with a degree of opacity around decisions and actions, and complex bureaucratic processes that can obstruct the smooth and timely payment of government obligations, it said.

"While Vietnam's large foreign exchange reserves and modest government financing requirements denote ample capacity to meet debt obligations, the review period will allow Moody's to ascertain if the revealed institutional weaknesses raise the risk of future delayed or missed payments that could denote weaker willingness to pay than Moody's has previously assessed.

But it said it expects Vietnam's credit profile to remain "underpinned by strong growth potential" and the government's debt burden to remain broadly stable at just under 50 percent of GDP given the absence of significant economic or contingent liability shocks.

It expects to complete the review in three months.

Other credit ratings firms Fitch and Standards & Poors (S&P) have given Vietnam a BB rating, a notch higher than Moody’s Ba3, but both levels are described as "non-investment grade", meaning the probability that the issuer would repay the issued debt is deemed speculative.

Latest figures from the Ministry of Finance estimates public debt at the end of 2018 to be 58.4 percent of GDP, or $136.75 billion, the lowest since 2015, and well below the 63.9 percent target set by the Ministry of Planning and Investment last August.

But National Assembly delegates warned in June that Vietnam might need to borrow around VND700 trillion ($30 billion) for three years to service its debts since repayment pressure is rising.