May 21 (Reuters) - Mars tumbled on Wednesday to four-month lows, dealers said, the first day of roll trade and as investors worried about demand for the oil amid increased OPEC+ output and reports that the U.S. government is set to extend Chevron's CVX.N deadline to halt its operations in Venezuela by another 60 days.

Mars swung to a discount of 30 cents to U.S. West Texas Intermediate crude, the lowest in four months. WTI Midland and WTI at East Houston WTC-MEH, also known as MEH, eased 10 cents each.

The three-day roll trade begins on Wednesday after the front-month U.S. crude futures contract CLc1 expires. Traders use the period to square positions and manage exposure.

The cash roll from June to July traded at 75 cents.

OPEC+ has agreed to accelerate oil production hikes for a second consecutive month, raising output in June by 411,000 barrels per day, the group said on Saturday, despite falling prices and expectations of weaker demand.

U.S. President Donald Trump's administration is set to extend Chevron's CVX.N deadline to halt its operations in Venezuela by another 60 days as the U.S. continues negotiations with President Nicolas Maduro's government over the fate of U.S. detainees in the country, Bloomberg News reported on Tuesday, citing a person familiar with the matter.

Meanwhile, U.S. crude rose by 1.3 million barrels to 443.2 million barrels in the week ended May 16, the Energy Information Administration said on Wednesday, as crude imports hit a six-week high.

* Light Louisiana Sweet WTC-LLS for June delivery was flat at a midpoint of a $2.50 premium and was seen bid and offered between a $2.25 and $2.75 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS eased $1 to a midpoint of a 30-cent discount and was seen bid and offered between a 50-cent and 10-cent a barrel discount to U.S. crude futures CLc1

* WTI Midland WTC-WTM eased 20 cents to a midpoint of a 55-cent premium and was seen bid and offered between a 40-cent and 70-cent a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS was seen bid and offered between a 90-cent and 50-cent a barrel discount to U.S. crude futures CLc1

* WTI at East Houston WTC-MEH, also known as MEH, traded between a 50-cent and 90-cent a barrel premium to U.S. crude futures CLc1

* ICE Brent July futures LCOc1 fell 47 cents to settle at $64.91 a barrel on Wednesday.

* WTI July crude CLc1 futures settled 46 cents lower at $61.57 a barrel on Wednesday.

* The Brent/WTI spread WTCLc1-LCOc1 narrowed 4 cents to last trade at minus $3.31, after hitting a high of minus $3.28 and a low of minus $3.36.

(Reporting by Arathy Somasekhar in Houston;)

((arathy.s@tr.com))