The head of the assembly that is rewriting Ecuador's constitution offered to resign on Monday, exposing a rift in the leftist government as it seeks to overhaul institutions to bolster the president's power.

President Rafael Correa has prioritized passing a new constitution this year which would allow him to run for office again and would rewrite the rules for the vital mining and oil industries.

But assembly chief Alberto Acosta, a former oil minister who generally wants more curbs on foreign investment in mining than the president, told Reuters the five weeks remaining to rewrite the constitution was too little.

He quit because his party refuses to grant him an extension. It is unclear if the assembly, where the government has a clear majority, will accept his resignation.

This is irreversible for the moment, he said in a telephone interview. I do not have the party leadership's support.

Correa has criticized the increasingly unpopular assembly, which Ecuadoreans perceive is doing too little work.

Once the assembly produces its proposed constitution, Ecuadoreans will vote on it in a referendum currently scheduled for September.

Correa still has much work to do to convince Ecuadoreans to pass the constitution but his high popularity gives him a good chance of winning the vote, pollsters say.

(Reporting by Alonso Soto and Enrique Andres Pretel; Writing by Saul Hudson; Editing by Brian Ellsworth)