American security officials say a paper shortage was a key reason the African nation of Chad was hit with travel restrictions by the US.

President Donald Trump's administration added Chad to a travel ban order last month, which a US federal judge put on hold this week on grounds that the order "plainly discriminates based on nationality".

But Chad's inclusion was perplexing to many because the country works closely with the US on fighting terrorism. American security services have praised Chad's cooperation on counter-terrorism, especially its campaign against a vicious Boko Haram insurgency spilling over from Nigeria.

However, it turned out a seemingly pedestrian issue was to blame – Chad had ran out of passport paper.

Earlier this year, the US Homeland Security Department had given all countries 50 days to take several steps, including providing a sample of the latest secure passport paper. The central African nation couldn't comply, and its offer to provide an existing sample wasn't sufficient.

This wasn't enough to persuade the department to make an exception to requirements it has been strictly applying to countries across the globe, unnamed US officials told the Associated Press.

US National security adviser H.R. McMaster has said that Chad could come off the list "maybe in a couple of months".

McMaster spoke to Chadian President Idriss Deby last week about getting the visa restrictions removed, the State Department said, but the country remains on the list.

The Homeland Security Department confirmed that the US "lacks a recent sample from Chad" of its passports, but said there were other problems, too.

"The restrictions placed on Chad dealt with more than just the receipt of a passport exemplar. Chad does not adequately share public safety and terrorism-related information," said Homeland Security spokesman David Lapan.

He added the US was working closely with Chad on the issue and was "eager to see Chad develop more secure travel documents and make other enhancements."

The new restrictions that were set to come into effect this week covers eight countries – Chad, Iran, Libya, North Korea, Syria, Venezuela, Somalia and Yemen.