An oil tanker and cargo ship are both reported to be on fire after colliding in the North Sea off the coast of Lincolnshire, the RNLI said.
The lifeboat service added that it had sent four crews to the scene after the collision was reported just before 9.50am.
There have been reports a number of people abandoned the vessels following the collision, the RNLI added.
A coastguard helicopter took off from Humberside and headed to the scene alongside lifeboats from Skegness, Bridlington, Maplethorpe and Cleethorpes, and an HM Coastguard fixed wing aircraft, and nearby vessels with fire-fighting capability.
The oil tanker is named Stena Immaculate and is sailing under a US flag, while the cargo ship is named Solong and is Portuguese-flagged, according to Marinetraffic.com.
The website also described the US-flagged vessel as being an "oil/chemical tanker".
HM Coastguard said earlier it is currently co-ordinating the emergency response.
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