Moody's switched the outlook on its B3 rating for Argentina from stable to negative, warning of haphazard economic policy and a lack of progress in debt talks.
A drop below B3 into the C rating range would indicate that Moody's believes Argentina is vulnerable to default.
"A lack of resolution to Argentina's debt arrears with Paris Club creditors, which has been pending for several years and shows no signs of an eventual resolution, raising fundamental questions about the sovereign's willingness to pay," noted Moody's.
Argentina has almost $9 billion in overdue debt to the Paris Club of government creditors.
Moody's said it also believed there would be a continued "haphazard policy environment, evidenced most recently in the nationalisation without compensation of YPF, Argentina's largest oil company, and import controls that have stalled economic growth."
Rival ratings agency Standard & Poor's lowered the outlook on its B rating for Argentina in April after it stripped Spain's Repsol of its controlling stake in YPF.
Moody's also warned of growing concerns about the reliability of Argentina's official economic data, noting that underreporting of inflation is of concern as a fifth of the country's debt is indexed to inflation.
It also warned a large and sustained drop in commodity prices would also affect its view of the major commodity exporter's economic and debt repayment perspectives.