The US Supreme Court has cleared the way for President Donald Trump's travel ban.

Key points: The ban is effective immediately

The ban is effective immediately It affects travellers from Chad, Iran, Libya, Syria, and Yemen.

It affects travellers from Chad, Iran, Libya, Syria, and Yemen. Two US courts will be holding arguments on the ban's legality this week.

It was already in partial effect, but a court order prevented it from being applied to people with certain connections to the US.

That order has now been lifted, meaning relatives can be prevented from visiting family in the US.

The ruling will take effect immediately, despite ongoing challenges to it in US lower courts.

Who does it affect?

The ban applies to travellers from six Muslim-majority countries: Chad, Iran, Libya, Somalia, Syria and Yemen.

Mr Trump's ban also covers people from North Korea and certain government officials from Venezuela, but the lower courts had already allowed those provisions to go into effect.

Lower courts had said people from those nations with a claim of a "bona fide" relationship with someone in the United States could not be kept out of the country. Grandparents, cousins and other relatives were among those the courts said could not be excluded.

Certain people from each targeted country can still apply for a visa for tourism, business or education purposes, and any applicant can ask for an individual waiver.

Wasn't there already a ban?

Mr Trump issued his first travel ban targeting several Muslim-majority countries in January, then issued a revised order in March after the first was blocked by federal courts.

The second one expired in September after a long court fight and was replaced with the present version.

Hundreds demonstrated at LA airport after Donald Trump first issued the travel ban. ( AP: Reed Saxon )

This time, the nine-member Supreme Court, with two liberal justices dissenting, granted his administration's request to lift two injunctions imposed by lower courts that had partially blocked the ban.

The ban was challenged in separate lawsuits by the state of Hawaii and the American Civil Liberties Union (ACLU).

Both sets of challengers said the latest ban, like the earlier ones, discriminated against Muslims in violation of the US Constitution and was not permissible under immigration laws.

What happens next?

Courts in San Francisco and Virginia will be holding arguments on the legality of the ban this week.

Trump travel ban timeline: January 27: Donald Trump signs the first executive order

January 27: Donald Trump signs the first executive order February 3: A Federal judge temporarily halts the key provisions of the order

February 3: A Federal judge temporarily halts the key provisions of the order February 9: The travel ban remains blocked

February 9: The travel ban remains blocked March 6: A new travel ban is unveiled

March 6: A new travel ban is unveiled March 15: The ban is blocked again

March 15: The ban is blocked again June 26: US Supreme Court revives part of the travel ban

June 26: US Supreme Court revives part of the travel ban September 24: Expanded ban issued to include North Korea, Venezuela

September 24: Expanded ban issued to include North Korea, Venezuela October 17: Federal judge in Hawaii blocks latest ban

October 17: Federal judge in Hawaii blocks latest ban December 4: US Supreme Court allows travel ban to go into full effect

But the Supreme Court said the ban will remain in effect regardless of what the appeals courts rule, at least until the justices ultimately decide whether to take up the issue again, which they are highly likely to do.

Both courts are dealing with the issue on an accelerated basis, and the Supreme Court noted it expected those courts to reach decisions "with appropriate dispatch."

Quick resolution by appellate courts would allow the Supreme Court to hear and decide the issue by the end of June next year.

Why does Trump want the ban?

Mr Trump had promised as a candidate to impose "a total and complete shutdown of Muslims entering the United States".

The Trump administration said the president put the latest restrictions in place after a worldwide review of the ability of each country to issue reliable passports and share data with the US.

The administration has said the ban is not discriminatory and pointed out that many Muslim-majority countries were unaffected by it. It also argued that a president has broad authority to decide who can come into the US.

But detractors said the expanded ban violated a law forbidding the government from discriminating based on nationality when issuing immigrant visas.

Reuters/AP/ABC