American delivery firms UPS and FedEx have temporarily grounded part of their fleets of cargo planes after a mid-take-off crash in Kentucky on Tuesday left at least 14 people dead.

The UPS aircraft burst into flames shortly after take-off when it collided with neighbouring business premises, triggering a huge fire that gutted several buildings and closed Louisville International Airport.

UPS said the decision to ground MD-11 models followed instructions from manufacturer Boeing, while rival delivery giant FedEx confirmed it would follow suit.

Officials are yet to determine what caused the deadly crash.

Hours before the fleet was grounded, officials confirmed the death toll had risen to 14 after another body was found, with buildings caught up in the fire still being searched for more victims.

"We pray for each of the victims' families and pray that no additional victims are lost," Louisville Mayor Craig Greenberg said.

In a statement, UPS said the decision to ground 9% of its fleet was out of an "abundance of caution".

Boeing later said in a statement to Reuters that it recommended UPS and FedEx suspend use of the MD-11 plane.

"This recommendation was made in an abundance of caution and we will continue coordinating with the FAA on this matter," a Boeing spokesperson said.

It is unclear to what extent delivery services will be impacted by the moves from UPS and FedEx, which are the world's largest cargo carriers.

Neither company has so far listed any active service disruptions as a result of the grounding of their MD-11 fleets.

FedEx said it operated 28 MD-11s and Reuters quoted a UPS factsheet that said the firm had 27 in its fleet before the Kentucky crash.

"Contingency plans are in place to flex our integrated air and ground network to help protect time-sensitive, critical shipments such as lifesaving pharmaceuticals, medical devices, and other products move through our network without disruption," FedEx said in a statement.

UPS also said contingency plans were in place to continue its service. A statement continued: "Nothing is more important to us than the safety of our employees and the communities we serve."

The BBC has contacted Western Global Airlines - another US cargo airline that also uses MD-11s.

The model of aircraft involved in the crash was an MD-11F triple-engine plane, which first entered service 34 years ago with Thai Airways as a passenger jet, but was transferred to UPS in 2006.

MD-11s were originally manufactured by McDonnell Douglas, before the company merged with Boeing in 1997. Production of MD-11 planes ended in 2000, while passenger service officially ended in 2014.

According to the National Transportation and Safety Board, the left engine caught fire and detached from the wing during take-off.

The aircraft was carrying 38,000 gallons (144,000 litres) of fuel as it took off for a flight to Hawaii, which contributed to the size of the fire after the crash. (BBC)