US President Donald Trump has expanded a travel ban by imposing full entry restrictions on nationals from six more countries, including Syria, and holders of Palestinian Authority-issued travel documents, the White House said.

In a social media post, a White House account said Trump was acting "to protect the security of the United States," as part of his long-standing campaign to tighten immigration controls.

The White House said Trump had signed a proclamation further restricting and limiting the entry of foreign nationals to the United States.

Under the measure, full restrictions and entry limitations have been imposed on nationals from Burkina Faso, Mali, Niger, South Sudan, and Syria, in addition to an earlier list of 12 countries already subject to restrictions.

The proclamation also applies full entry restrictions to individuals holding travel documents issued by the Palestinian Authority, the White House said.

Trump has repeatedly argued that stricter immigration policies are necessary to safeguard US national security and has made border control and travel restrictions a central part of his political platform.

The White House did not provide details on how long the new restrictions would remain in place or whether exemptions would be granted in specific cases.

Trump has previously imposed and expanded travel bans during his presidency, measures that have drawn criticism from rights groups and legal challenges, while being defended by the administration as lawful and necessary.