BELGRADE : Serbian authorities have found two US-made guided Hellfire missiles inside wooden crates at Belgrade airport bound for Portland, Oregon, in the United States and are investigating them, a source at the prosecutor's office said on Monday.

The missiles had arrived on an Air Serbia flight from the Lebanese capital Beirut and had been due to be transferred to another plane. They were discovered by bomb-sniffing dogs at Belgrade airport on Saturday, the source said. "Experts are determining whether the missiles were equipped with live or training warheads ... They were packed in proper transportation crates and supplied with paperwork which is also being scrutinised," the source said.

In Beirut, a Lebanese security source said they were training missiles used by the Lebanese army and were being sent back to the United States. The Lebanese and US authorities were aware of the shipment and the missiles posed no threat to the public, the source said.

The AGM 114 Hellfire, produced by Lockheed Martin, is an air-to-surface missile (ASM) which can be used against armoured vehicles and tanks, but also for precision strikes against other targets.

In addition to a version with a high-explosive warhead, the Hellfire is also produced as a practice weapon with a spotting charge instead of a warhead or with an inert rocket motor and warhead.

Air Serbia said it was helping with the investigation and that security and safety were its main priorities.

An inert AGM 114 Hellfire missile that had arrived in Cuba by mistake in 2014 was retrieved last month by US officials and Lockheed Martin representatives, the Cuban foreign ministry said.