(Reuters) -- The U.S. Navy sent a military aircraft to assist with the search, which spanned an area of about 3,600 square nautical miles (12,350 sq kms), but said in a statement it did not locate any of the tanker’s 32 missing crew members.

The Sanchi tanker run by Iran’s top oil shipping operator, collided with the CF Crystal about 160 nautical miles off China’s coast near Shanghai and the mouth of the Yangtze River Delta on Saturday evening.

Chinese state media CCTV showed pictures on Sunday of the tanker ablaze and billowing plumes of thick dark smoke. China sent four rescue ships and three cleaning boats to the site, while South Korea dispatched a ship and a helicopter.

The Panama-registered tanker was sailing from Iran to South Korea, carrying 136,000 tonnes of condensate, an ultra light crude. That is equivalent to just under 1 million barrels, worth about $60 million, based on global crude oil prices.

The freight ship, which was carrying U.S. grain, suffered limited damage and the 21 crew members, all Chinese nationals, were rescued.

The extent of the environmental harm and size of the spill were not yet known, but based on the tonnage the tanker was carrying, it has the potential to be the worst since 1991 when 260,000 tonnes of oil leaked off the Angolan coast.

Hanwha Total Petrochemical Co Ltd in South Korea was due to receive the cargo and was looking at ways to replace the lost barrels, a spokesman said.

The company may use its own stock, ask Iran for another cargo or seek alternative condensate supplies from Qatar, he said. The accident was not affecting its operations on Monday, he said.

COLOURLESS, ODORLESS AND HARMFUL

Bad weather on Sunday night made it hard for the rescue crews to get access to the tanker.

Trying to contain a spill of condensate, which is extremely low in density, highly toxic and much more explosive than normal crude, may also be difficult.

It is only liquid in certain pressure and temperature conditions and often evaporates into air or dilutes into water when exposed to the atmosphere or spills uncontrolled.

When liquid, condensate is colourless and virtually odourless. Surface spills of condensate are therefore difficult to detect visually, making them hard to manage and contain.

Tankers also carry shipping fuel, known as bunker, which is extremely heavy and toxic, when spilled, though much less explosive.

Shanghai Maritime Bureau’s navigation department said the collision did not affect impact traffic in and out of Shanghai, one of the world’s busiest and biggest ports, or ports along the Yangtze river.