US law enforcement is investigating an incident near a golf course as a possible assassination attempt against former president Donald Trump. The Republican presidential candidate is unharmed.

The Federal Bureau of Investigation and others say a man possessing a firearm was detained near the golf course where Trump was playing in the state of Florida on Sunday afternoon.

US media outlets say the 58-year-old man was involved in activities to support Ukraine.

ABC News reported that investigators are looking into whether he "had grievances related to Ukraine and specifically frustrations with Trump's position on this issue."

But the man's motives are not known.

Trump's Democratic rival, Vice President Kamala Harris, released a statement following the incident.

She said, "I am deeply disturbed by the possible assassination attempt of former President Trump today."

She also said, "I condemn political violence. We all must do our part to ensure that this incident does not lead to more violence."

US media are giving wide coverage to what they describe as the second assassination attempt against Trump in two months.

A bullet pierced his ear during a campaign rally in July. Attention is on how the latest incident will affect the presidential election in November.