Travellers to the United States from seven Muslim-majority countries singled out for "extreme vetting" will face scrutiny of their social media activity and phone records, the new US Homeland Security secretary has said.

John Kelly on Tuesday sought to explain President Donald Trump's travel ban, four days after he issued it with no warning, setting off mass protests, legal challenges and confusion.

"There are many countries, seven that we are dealing with right now, that in our view and my view don't have the kind of law enforcement, records-keeping, that kind of thing, that can convince us that one of their citizens is indeed who that citizen says they are," Kelly said in a press conference.

For that reason, he said, US authorities will investigate visa applicants' social media use and telephone contacts, "so that we can see who they are talking to".

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On Friday, Trump ordered a suspension of arrivals from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen, including refugees.

With the move under widespread criticism, Kelly denied it specifically targets Muslims, which could violate the US Constitution.

"The vast majority of the 1.7 billion Muslims that live on this planet, all other things being equal, have access to the United States," he said.

"And a relatively small number right now are being held up for a period of time until we can take a look at what their procedures are."

Longer ban possible

Trump's order halted immigration from the seven countries for at least 90 days, but Kelly suggested that for some the ban could go on longer if stronger vetting procedures are not in place once the review period has elapsed.

"Some of those countries that are on the list may not be taken off the list anytime soon. There are countries that are in various states of collapse, for example," Kelly said, without offering specifics.

The sudden order caught many US immigration gateways and foreign airlines by surprise, resulting in many people with legal US residency being blocked from boarding aircraft for the US, or being detained upon arrival.

US Customs and Border Protection Acting Commissioner Kevin McAleenan said travellers with dual nationality could enter the US as long as the passport they present is acceptable.

"Travellers will be assessed at our borders based on the passport that they present, not any dual national status," he said.

By Monday 721 people had been denied boarding, while more than 1,000 people were granted waivers from the Trump order to allow them to enter the country, McAleenan said.

Al Jazeera's Kimberly Halkett, reporting from Washington DC, said: "The White House maintains these are extreme vetting procedures, not a Muslim 'ban', not a travel 'ban'."

However, Trump on Monday tweeted: "If the ban were announced with a one week notice, the "bad" would rush into our country during that week. A lot of bad 'dudes' out there!"