Washington DC - President Donald Trump appeared at a White House event on Monday with a prominent red rash on his neck, which his doctor attributed to a "preventative" treatment.

An AFP photograph showed a red area with several brownish scabs protruding above his shirt collar on the right side of the neck of the 79-year-old president, the oldest ever elected.

"President Trump is using a very common cream on the right side of his neck, which is a preventative skin treatment, prescribed by the White House Doctor," presidential physician Sean Barbabella said in a statement to AFP.

"The president is using this treatment for one week, and the redness is expected to last for a few weeks."

The red patch was spotted during a Medal of Honor presentation ceremony, after a busy weekend in which Trump launched US military strikes on Iran.

Trump has previously been seen with bruises on his hands, which the White House attributed to aspirin use.

Speculation has swirled about Trump's health given the bruising, along with swelling in his legs, and several apparent moments of dozing off during public events.

The White House disclosed last summer that Trump's leg swelling had been diagnosed as a symptom of chronic venous insufficiency – a common condition in which faulty vein valves allow blood to pool, causing swelling, cramping, and skin changes.