Georgina McCartney, May 11 (Reuters) - Offshore grades rose on Monday, dealers said, as the war in Iran showed no signs of abating while domestic refinery demand looked set to rise.

U.S. President Donald Trump said the ceasefire with Iran was "on life support," leaving the Strait of Hormuz largely closed with no clear end in sight to the war.

On the demand side, U.S. oil refiners are expected to have about 414,000 barrels per day of capacity offline in the week ending May 15, increasing available refining capacity by 180,000 bpd from the previous week, research company IIR Energy said.

Offline capacity is expected to fall to 48,000 bpd in the week ending May 22, IIR said.

The U.S. government on Monday announced sanctions against three people and nine companies, including four based in Hong Kong and four in the United Arab Emirates, for aiding Iran's shipment of oil to China. The ninth company is based in Oman.

OPEC oil output dropped further in April to the lowest in more than two decades, a Reuters survey found, as the war with Iran effectively closed the Strait of Hormuz and forced export cuts.

Crude output by the 12-member Organization of the Petroleum Exporting Countries in April fell by 830,000 bpd month-on-month to 20.04 million bpd, the survey found. March's figure was revised 700,000 bpd lower due to a change in the Saudi estimate.

Three tankers carrying crude exited the Strait of Hormuz last week and on Sunday with trackers switched off to avoid Iranian attacks, shipping data from Kpler and LSEG showed on Monday, underscoring a rising trend to sustain Middle East oil exports.

• Light Louisiana Sweet for June delivery rose 87 cents to a midpoint of a $1.50 premium and was seen bid and offered between a $1.40 and $1.60 a barrel premium to U.S. crude futures CLc1

• Mars Sour rose $1 to a midpoint of a $1 premium and was seen bid and offered between a 90-cent and $1.10 a barrel premium to U.S. crude futures CLc1 ​

• WTI Midland fell 35 cents to a midpoint of a $1.80 premium and was seen bid and offered between a $1.70 and $1.90 a barrel premium to U.S. crude futures CLc1 ​

• West Texas Sour fell 52 cents to a midpoint of a $1.70 discount and was seen bid and offered between a $1.80 and $1.60 a barrel discount to U.S. crude futures CLc1

• WTI at East Houston, also known as MEH, traded between a $2.05 and $2.25 a barrel premium to U.S. crude futures CLc1 ​

• ICE Brent July futures LCOc1 rose $2.92 to settle at $104.21 a barrel

• WTI June crude CLc1 futures rose $2.65 to settle at $98.07 a barrel

• The Brent/WTI spread narrowed 5 cents to last trade at minus $9.45, after hitting a high of minus $9.16 and a low of minus $10.05