KHABAROVSK, Russia, July 9 (Reuters) - Russian rescue services were on Wednesday battling a fuel oil spill in the Amur River which forms part of Russia's border with China.

A major chemical spill from a Chinese plant into the same river network three years ago caused diplomatic tensions between Moscow and Beijing.

Russian border guards on Tuesday spotted the 2 km (1.2 mile) slick on a tributary of the Amur river, which is known in China as Heilong, before it flowed downstream.

"More than 500 kg of fuel oil has entered the river," Russia's Natural Resources Ministry said in a statement.

The slick is expected to reach Khabarovsk, a city of 580,000, in mid-July. Local officials said there were no drinking water reservoirs along the route.

"We initially thought the fuel came from a ship of some sort but that was not confirmed and we are still searching for the source," Yuri Ivanov, deputy head of the local emergency ministry, told reporters in Khabarovsk.

Ivanov said rescuers had been unable to place oil spill booms across the river as the other bank is Chinese territory.

An explosion in November 2005 in China poured 100 tonnes of cancer-causing benzene compounds into the Songhua river, which flows into the Amur. (Writing by Guy Faulconbridge, Editing by Janet Lawrence)

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