SINGAPORE, Feb 24 (Reuters) - Supply disruptions at Qatar's largest offshore oilfield al-Shaheen are expected to last until April as at least three lifters have been notified of loading delays in February and March, three trade sources said on Friday.

The crude is one of four grades approved for delivery in trades that are carried out during the Platts Market on Close assessment process. Trades on the platform, commonly known as the window, are used to assess the Middle East crude benchmark price Dubai.

Loading dates for cargoes in February and March have been pushed back by 5-7 days on average, the sources said.

"Some of the cargoes are slipping into April," one of the sources said.

The field operated by Maersk Oil, a unit of Danish shipping and oil group A.P. Moller-Maersk, produces 300,000 barrels per day.

"We do not comment on the day to day operations or lifting schedules from our business units," said a Maersk Oil spokesman.

A spokesman at Qatar Petroleum declined to comment.

Al-Shaheen supplies have become erratic since late last year, trade sources said.

Exports fell in September-October and also in January due to maintenance work. In April, the number of cargoes due to be exported is three less than in a typical month.

QP announced in June that Total will take over from Maersk as the field's operator from July 2017.

(Reporting by Florence Tan in SINGAPORE, Tom Finn in DOHA and Stine Jacobsen in COPENHAGEN; Editing by Richard Pullin) ((Florence.Tan@thomsonreuters.com; +65 6870 3497; Reuters Messaging: florence.tan.thomsonreuters.com@reuters.net))