May 28 (Reuters) - U.S. coastal crude grades strengthened on Wednesday, dealers said, while domestic refiners are expected to increase their capacity and a major refinery in Joliet, Illinois brought a unit back to full capacity.

Mars Sour WTC-MRS strengthened 45 cents at a midpoint of a 95-cent premium. Wednesday's trading session was the fourth consecutive session of gains for the grade.

U.S. oil refiners are expected to have about 229,000 barrels per day of capacity offline in the week ending May 30, increasing available refining capacity by 169,000 bpd, research company IIR Energy said on Wednesday.

In Illinois, Exxon Mobil's XOM.N 250,000 barrel-per-day refinery in Joliet, Illinois brought the catalytic cracker unit back to full capacity, IIR Energy said on Thursday.

Coastal grades strengthened despite a narrowing discount of U.S. crude futures CLc1 versus the international benchmark Brent LCOc1. The discount traded at as narrow as minus $2.99 a barrel on Wednesday, the narrowest since late March.

Typically a narrower spread makes U.S. grades less attractive to foreign buyers.

Abroad, OPEC+ agreed to leave their output policy unchanged.

* Light Louisiana Sweet WTC-LLS for July delivery strengthened 15 cents at a midpoint of a $2.15 premium and was seen bid and offered between a $2.00 and $2.30 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS strengthened 45 cents at a midpoint of a 95-cent premium and was seen bid and offered between a 85-cent and $1.05 a barrel premium to U.S. crude futures CLc1

* WTI Midland WTC-WTM weakened 5 cents at a midpoint of a 60-cent premium and was seen bid and offered between a 50-cent and 70-cent a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS was little changed at a midpoint of a parity and was seen bid and offered between a discount of 10 cents and 10-cent a barrel premium to U.S. crude futures CLc1

* WTI at East Houston WTC-MEH, also known as MEH, traded between a 70-cent and 90-cent a barrel premium to U.S. crude futures CLc1

* ICE Brent July futures LCOc1 rose 81 cents to settle at $64.9 a barrel

* WTI July crude CLc1 futures rose 95 cents to settle at $61.84 a barrel

* The Brent/WTI spread WTCLc1-LCOc1 narrowed to last trade at minus $3.05, after hitting a high of minus $2.99 and a low of minus $3.21.

(Reporting by Stephanie Kelly in New York; Editing by Cynthia Osterman)

((Stephanie.Kelly@tr.com))