"The slick is about 500 litres. It’s maybe 15 kilometres (9 miles) long," said a dispatcher from the Czech agency that oversees the Elbe.
The oil slick was discovered at 8.30 a.m. local time (0730 GMT) and officials quickly began enacting a plan to control it, he added, giving no further details.
The Ministry of Environment and Agriculture in the eastern German state of Saxony said it had been informed by the Czechs of the oil slick, which is in the vicinity of Usti nad Labem, a city near the northern Czech border with Germany.
"The cause of the contamination is not yet clear," the ministry said in a statement. "It is possible that it was a damaged pipeline or a freighter or truck accident."
"The Czechs have erected barriers that should slow down the spread of the oil slick. It is not clear whether and when the oil slick will reach Saxony," it said.
The Elbe flows up from the Czech Republic, where it is known as the Labe River, through Dresden and up north to Hamburg.