Washington: President Donald Trump said he would wait for the courts to rule before acting on raising the minimum age for some gun purchases, putting off one of the more contentious gun safety measures he had backed after the latest US school shooting.

The proposal to raise the minimum age for buying guns from 18 to 21 was not part of a modest set of gun safety proposals announced on Sunday night by Trump administration officials, which included training teachers to carry guns in schools and improving background checks.

"On 18 to 21 age limits, watching court cases and rulings before acting. States are making this decision. Things are moving rapidly on this, but not much political support (to put it mildly)," he wrote on Twitter.

Trump has said he believes armed teachers would deter school shootings and better protect students when they happen. The idea, already in place in some states, is backed by the National Rifle Association gun lobby.