Donald Trump's military is lining up battlefield surgeons, military police and combat engineers for a potential operation against Cuba within the next four months.

US Army Reserve headquarters sent an internal message last week asking its commands whether they could hand six types of units over to Southern Command, which oversees American forces in Latin America and the Caribbean.

The memo does not mention Cuba, but multiple US officials said the planning is aimed squarely at the communist island, according to CBS News, which obtained the document.

The units would 'possibly [be] needed in 90 to 120 days,' the memo states, opening a potential deployment window around Christmas. Commands were given until Friday to respond.

Among the units requested is a forward resuscitative and surgical detachment, a small team that performs emergency surgery and trauma care on wounded troops close to the fighting.

There is no indication any units have been given deployment orders, and the message does not say how many troops would be needed, where they would go or what operation they would support.

Trump told the United Nations General Assembly on Tuesday that Cuba's communist government 'will fall' and 'freedom will be coming,' after US special forces snatched Venezuelan dictator Nicolas Maduro from Caracas in January.

The Cuban delegation stormed out of the hall as the President made the veiled threat. Trump added that Secretary of State Marco Rubio was 'deep into negotiations' with Havana.

The memo also asks for a medical brigade to direct units in the field, and a military police brigade, whose units typically handle security, detention and law enforcement missions.

Also on the list are an engineer battalion, a logistics battalion that manages transport, fuel, maintenance and supplies, and an expeditionary sustainment command to run logistics across an entire theater of operations.

A Southern Command spokesman said: 'Due to operational security concerns, we do not disclose details or comment on US operational movements or potential deployment timelines associated with current or future operations, contingencies, or planning.

'However, US Southern Command continuously coordinates with the Joint Force to assess a range of potential requirements across its [area of responsibility] in support of assigned missions.'