By Hilary Clarke

(CNN) -- US President Donald Trump described Venezuelan President Nicolas Maduro as a "bad leader" and an aspiring "dictator" on Monday, and threatened to take "strong and swift economic actions'' if Maduro goes ahead with plans to rewrite the country's constitution.

Maduro is pushing a plan to create a new Constituent Assembly with the power to rewrite Venezuela's constitution. The body would replace the National Assembly, which is currently controlled by Maduro's opponents. A nation-wide referendum is due to be held on the issue at the end of the month.

On Sunday 7.2 million opposition supporters, almost a third of the country's registered electorate, voted in an unofficial non-binding to reject the proposed assembly. The opposition is also calling for fresh elections before Maduro's term ends in 2019.

"Yesterday, the Venezuelan people again made clear that they stand for democracy, freedom, and rule of law," Trump said in a statement released Monday.

"Yet their strong and courageous actions continue to be ignored by a bad leader who dreams of becoming a dictator. The United States will not stand by as Venezuela crumbles. If the Maduro regime imposes its Constituent Assembly on July 30, the United States will take strong and swift economic actions," Trump said.

"The United States once again calls for free and fair elections and stands with the people of Venezuela in their quest to restore their country to a full and prosperous democracy."

Economic sanctions on Venezuela would have a devastating impact, as the country's oil-dependent economy is already reeling from the impact of low oil prices.

"There have been US sanctions on Venezuela but they were just targeted at certain individuals with asset freezes and stopping them going to the US, rather than sanctions on oil exports which would be much more explosive," said Edward Glossop, Latin America economist with the London-based economic think tank Capital Economics.

"There is no real indication that the US is willing to go as far as sanctioning the oil industry because it's not clear who that hurts. It might hasten the end of Maduro's regime but would also definitely make the humanitarian crisis worse in the near-term at least," he said.

According to the International Monetary Fund, the US is still the biggest market for Venezuelan oil. Venezuela also has the largest oil reserves in the world yet imports refined oil because the oil in Venezuela is very heavy and must be mixed with lighter oil grades to create petroleum.

Almost 100 people have been killed in popular protests against the proposed constitutional reforms in Venezuela and economic hardship that has been compounded by low oil prices.