Executive order sets sanctions, travel bans and export controls on states accused of "wrongful detention" of US citizens.

US President Donald Trump has signed an executive order creating a blacklist of countries that Washington says unjustly detain Americans, introducing sanctions and travel bans against states accused of engaging in "hostage diplomacy."

The order allows the State Department to designate nations as "state sponsors of wrongful detention," modelled after the terrorism sponsor list, with penalties that include sanctions, restrictions on US exports, and entry bans on officials involved in imprisonments.

"With this EO you are signing today, you are drawing a line in the sand that US citizens will not be used as bargaining chips," Trump aide Sebastian Gorka said in the Oval Office.

While no countries were named immediately, a senior official said China, Iran and Afghanistan are under review as governments that "persistently participate in hostage diplomacy."

In an unprecedented step, the executive order authorises the US to bar its own citizens from travelling to countries placed on the blacklist.

Washington has previously imposed such restrictions only on North Korea, after the death of American student Otto Warmbier in 2017 following his detention there.

The blacklist can also apply to groups that control territory but are not recognised as states.