S&P Global Ratings said Wednesday it has lowered Venezuela's sovereign debt ratings, warning of the risk of default as economic conditions in the oil-rich country worsen and political tensions rise.

Venezuela has been wracked by more than a month of violent anti-government street protests that have killed nearly 40 people and left hundreds injured and arrested.

Opposition forces are calling for the removal of President Nicolas Maduro, blaming him for an economic crisis that has saddled the country with major shortages of food, medicine and other basic goods.

The ratings agency said it had lowered "long-term foreign and local currency sovereign credit ratings on the Bolivarian Republic of Venezuela to 'CCC-' from 'CCC'". 

The downgrade "reflects continued deterioration in economic conditions, rising political tensions, including within the Chavista government base, and the government's worsening liquidity position.

"Economic policy continues to be erratic, in a context of very high inflation; severe fiscal, monetary, and exchange-rate rigidities; and draconian controls on imports."

Recent developments had "raised the risk of default," S&P said, adding that it was concerned that Venezuela, which has been feeling the pinch from low oil prices for a number of years, may not be able to finding external funding and subsequently be unable to repay its debt. 

"We expect the government to have difficulty paying debt service of around $2.8 billion in the second half of 2017 and around $7 billion in 2018," it said, forecasting a contraction in the country's gross domestic product of around six percent this year.