* Shell is one of the biggest foreign investors in Singapore
* Singapore is Asia's main oil trading hub
* Southeast Asia is seen as a hotspot of illegal oil trading (Adds detail on Singapore oil trading hub; Southeast Asia illegal oil trading)
SINGAPORE, Jan 9 (Reuters) - Seventeen men have been arrested and millions of dollars in cash seized as part of an investigation into a suspected oil theft at Shell's biggest refinery, Singapore police said on Tuesday.
The arrests, made during raids on Sunday, come after Royal Dutch Shell Plc reported the theft to Singapore authorities at its Pulau Bukom industrial site in August last year.
The company said in a news release that the arrests included "a limited number of Shell employees" and that it anticipated "a short delay in the supply operations at Bukom." Those arrested, all men, ranged in age from 30 to 63.
Police said they also seized S$3.05 million ($2.29 million) in cash and a small, 12,000-deadweight-tonne tanker.
They have also frozen the suspects' bank accounts, the police said.
Bukom is the largest wholly owned Shell refinery in the world in terms of crude distillation capacity, according to the company's website.
Shell declined to say how much oil had been stolen. Shipping and oil refining have contributed significantly to Singapore's rising wealth during the past decades.
The Southeast Asian city-state is one of the world's most important oil trading hubs, with most of the Middle East's crude oil passing through Singapore before being delivered to the huge consumers in China, Japan and South Korea.
Singapore is also Southeast Asia's main refinery hub and the world's biggest marine refueling station.
Shell is one of the biggest and longest established foreign investors in Singapore. Its oil refinery on Bukom island, situated 5.5 km to the southwest of Singapore, is the company's biggest such facility in the world, with a processing capacity of 500,000 barrels per day.
Southeast Asia is a hotspot of illegal oil trading. In some cases, oil has been illegally siphoned from storage tanks, but there have also been thefts at sea.
The Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP) says that siphoning of fuel and oil at sea in Asia, including through armed robbery and piracy, saw sharp increases between 2011 and 2015.
There has been a modest decline since then, although the organisation said in a quarterly report that oil theft was still of concern, especially in the South China Sea, off the east coast of Malaysia.
The stolen fuel is generally sold across Southeast Asia, offloaded directly into trucks or tanks at small harbors away from oil terminals.
($1 = 1.3320 Singapore dollars)
(Reporting by John Geddie and Henning Gloystein; Additional reporting by Florence Tan; Editing by Gerry Doyle)