US President Donald Trump's immigration and travel ban made "an awful lot of sense" but probably should have been delayed at least long enough to brief Congress about it, homeland security secretary John Kelly has told politicians.

Mr Kelly's comment to the House Homeland Security Committee was the most direct acknowledgement by a high-level administration official that the rollout of Mr Trump's executive order had been mishandled.

"In retrospect, I should have - this is all on me, by the way - I should have delayed it just a bit so that I could talk to members of Congress, particularly to the leadership of committees like this, to prepare them for what was coming," Mr Kelly said in his first public meeting with politicians since being confirmed by the senate last month.

Mr Trump's executive order temporarily stopped citizens of seven Muslim-majority nations from entering the US and also temporarily barred the admission of refugees.

A court has blocked the order, but the administration is appealing.

Mr Kelly defended the order, saying it will enhance public safety "for all our citizens".

He was put on the defensive by Democratic politicians who have argued that the travel ban is inhumane, counterproductive and essentially a Muslim ban - an allegation Mr Kelly repeatedly denied.

Mr Kelly referred to the order as a "pause" that would give the US government time to fully evaluate how would-be visitors and refugees are being vetted before they are allowed into the country.

The Trump administration, including justice department lawyers defending the order in a federal appeals court, has said the travel ban was necessary to keep would-be terrorists out of the country.

Mr Trump has repeatedly tweeted that a court order temporarily blocking the ban is leading to "people pouring in".

In a tweet this week, Mr Trump said "many very bad and dangerous people may be pouring into our country".

Pressed by Representative Bennie Thompson, the committee's ranking Democrat, to address the president's claim, Mr Kelly said only that the government will not know for sure if someone with bad intentions entered the US "until the boom".

"We won't know until then," Mr Kelly said, referring to a possible attack.