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BELGRADE, Serbia — Authorities in Serbia have launched an investigation after two missiles bound for the U.S. were found in the unloaded cargo from a passenger plane, officials said Monday.

Belgrade prosecutor's spokesman Ivan Markovic said an Air Serbia flight transported the missiles from the Lebanese capital of Beirut to Belgrade on Saturday.

The missiles' documentation said their final destination was Portland, Oregon, Markovic told NBC News.

He said Serbian investigators were working to determine whether the missiles were active weapons or inert devices used for training purposes.

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A Lebanese security source told Reuters that the rockets were indeed training missiles being sent back to the U.S. after use by the Lebanese army. The source told the news agency that Lebanese and U.S. authorities were aware of the shipment and the missiles posed no threat to the public.

However, a senior police source in Belgrade told NBC News on condition of anonymity that the missiles had been discovered by a sniffer dog.

They were being transported inside wooden crates that were removed from the Air Serbia flight to be transferred to another aircraft, a Belgrade prosecutor's office source told Reuters.

Neither the U.S. Embassy in Belgrade nor Air Serbia immediately responded to NBC News' request for comment Monday.