(Reuters) - China's oil refinery output slipped 1.8% from year-ago levels in May, statistics bureau data showed on Monday, as refiners undertook planned maintenance overhauls and processing margins were pressured by rising crude costs.

Refiners processed 60.52 million metric tons of crude oil in May, according to data from the National Bureau of Statistics (NBS), equivalent to 14.25 million barrels per day (bpd).

This was down from 14.3 million bpd in April and 14.6 million bpd in May 2023.

Large state-run refineries such as Sinopec's Zhenhai and Zhanjiang, PetroChina's Dushanzi and Dalian plants, as well as big private refiner Hengli Petrochemical, were undertaking regular maintenance in May, according to Chinese commodities consultancy Oilchem.

Operational levels at smaller independent processors edged up last month to 55.53% of their capacity, estimated by Oilchem, up 0.75 percentage points from April as some plants were encouraged by slight margin improvements.

But it was still sharply lower than 62.2% a year earlier, reflecting higher crude costs and lagging domestic fuel prices, despite a brief spike in fuel demand during the first week of May for holiday travel as car and rail trips surged.

The number of domestic flights fell 1.79% year-on-year last month to 778,210 but were up 8.26% from pre-pandemic levels in 2019, according to civil aviation data provider Variflight. International flights roughly doubled from May 2023 to 58,878, although they remained 28.74% lower than in May 2019, the data showed.

Refinery throughput for January to May was up just 0.3% from a year earlier at 301.77 million metric tons, or 14.49 million bpd, the data showed.

NBS data showed China's crude oil production in May rose 0.6% from a year earlier to 18.15 million tons, or about 4.27 million bpd. From January to May, output grew 1.8% on the year at 89.1 million tons, or 4.28 million bpd.

Natural gas production rose 6.3% last month on the year to 20.3 billion cubic metres (bcm), and output between January and May gained 5.2% to 103.3 bcm.

(Metric ton = 7.3 barrels for crude oil conversion)

(Reporting by Colleen Howe and Chen Aizhu; Editing by Jamie Freed)