President Donald Trump has signed a new executive order placing a 90-day ban on people from six mainly Muslim nations.

Iraq - which was covered in the previous seven-nation order - has been removed from the new one after agreeing to additional visa vetting measures.

The directive, which includes a 120-day ban on all refugees, takes effect on 16 March.

The previous order, which was blocked by a federal court, sparked confusion at airports and mass protests.

Presented as a means to strengthen national security against terror threats, it was blocked by the courts and effectively remains on hold.

The new order was unveiled by Secretary of State Rex Tillerson, Attorney General Jeff Sessions and Department of Homeland Security Secretary John Kelly.

"The fact remains that we are not immune to terrorist threats and that our enemies often use our own freedoms and generosity against us," said Mr Kelly.

In justifying the refugee ban, Mr Sessions said there are more than 300 refugees under investigation for potential terror offences. But no further details were given.

What is different about the new order?

Citizens of Iran, Libya, Syria, Somalia, Sudan and Yemen, the other six countries on the original 27 January order, will once more be subject to a 90-day travel ban.

Iraq has been taken off the banned list because its government has boosted visa screening and data sharing, White House officials said.

The new directive says refugees already approved by the State Department can enter the US. It also lifts an indefinite ban on all Syrian refugees.

Green Card holders (legal permanent residents of the US) from the named countries will not be affected.

The new order does not give priority to religious minorities, unlike the previous directive.

Critics of the Trump administration had argued that this was an unlawful policy showing preference to Christian refugees.