President Donald Trump escalated his attacks Monday on a group of four minority congresswomen known as "the Squad," calling them "very Racist" and "not very smart."

Trump's assessment came in a tweet as his motorcade traveled from the White House to the Supreme Court to pay his respects to the late Justice John Paul Stevens, who died last week at age 99 and was lying in repose.

It was the latest in a string of attacks directed at the four freshman lawmakers since a week ago Sunday, when Trump said in a tweet that they should "go back" to the "totally broken and crime infested places from which they came." Only one of the four, Ilhan Omar, was born outside the United States, and she became a U.S. citizen in 2000.

Trump has often tried to turn the tables on his political opponents, accusing them of the very shortcomings for which they criticise him.

"The 'Squad' is a very Racist group of troublemakers who are young, inexperienced, and not very smart," Trump wrote. "They are pulling the once great Democrat Party far left, and were against humanitarian aid at the Border . . . And are now against ICE and Homeland Security. So bad for our Country!"

Over the past week, Trump has repeatedly defended his words directed at the four women - Omar, Alexandria Ocasio-Cortez of New York, Ayanna Pressley of Massachusetts and Rashida Tlaib of Michigan - despite widespread criticism that his remarks were racist and divisive.

Trump went on the offensive against the four lawmakers again Monday during a meeting in the Oval Office with Pakistani Prime Minister Imran Khan.

"I think they're very bad for our country. I really think they must hate our country," Trump told reporters.

President Donald Trump speaks to the media as he arrives on Air Force One at Pitt Greenville Airport, in Greenville. Photo / AP

He denied that he had created any "racial tension" in the United States and pointed to lower unemployment figures for African Americans.

During a heated exchange on "Fox News Sunday," Stephen Miller, a White House senior adviser, sought to defend Trump, saying that the term "racist" is being misused.

"I think the term 'racist' has become a label that is too often deployed by the left, Democrats, in this country simply to try to silence and punish and suppress people they disagree with, speech that they don't want to hear," Miller said.

Asked Monday about Trump's tweet calling the four lawmakers "racist," White House counselor Kellyanne Conway told reporters at the White House: "Well, they certainly are young and inexperienced. That doesn't stop all of you from elevating them into the stratosphere and superstardom."

Behind the scenes, Trump's staff and his allies continued to promote talking points that elevated the lawmakers, while avoiding directly injecting race into the arguments.

At a weekly meeting of Senate Republican communications aides, a White House official, Brad Bishop, encouraged the GOP staffers to emphasize a fresh message focusing on the four Democrats, according to two people in attendance: that the liberal lawmakers need to start helping their constituents, rather than focusing on unpopular issues such as impeaching Trump or abolishing Immigration and Customs Enforcement.

In a set of talking points distributed Monday, the president's reelection campaign encouraged allies to talk up Trump's willingness to defend the United States if questioned about his continued attacks on the "Squad."

"The President loves America. He will stick up for this country, our flag, and the men and women who serve this country in uniform," the talking points read, according to a copy obtained by The Washington Post. "He will also publicly oppose those who compare this country to garbage, show weakness - if not outright support - for terrorists, and make anti-Semitic remarks and level attacks on our closest ally in the Middle East - Israel."

The Republican National Committee also sent several suggested talking points, encouraging allies to highlight the liberal views of Ocasio-Cortez, Omar, Pressley and Tlaib and adding: "The despicable rhetoric espoused by the socialist squad and tolerated by national Democrats is beyond disgusting."

Trump has attacked the women on multiple issues in the past week, including over their views on Israel.

His mention of "humanitarian aid" on Monday referenced a US$4.6 billion border bill passed by the House late last month. The measure would pump billions of dollars into the budgets of agencies, including the Department of Health and Human Services, that have been overwhelmed by the influx of Central American migrants at the United States' southern border.

Ocasio-Cortez, Omar, Pressley and Tlaib voted against the legislation, saying it did not include enough restrictions on how the Trump administration could spend the money and did not include adequate protections for migrant children at detention facilities.

The lawmakers have also been highly critical of Immigration and Customs Enforcement on a range of issues.

During an appearance Monday morning at an NAACP convention in Detroit, Tlaib was introduced as "one of the four women who was told to go back home."

"Yeah, I'm not going nowhere, not until I impeach this president," she said upon taking the stage.

Last week, Trump directed most of his ire at Omar, a Somali-born refugee. As he criticised her during remarks at a campaign rally in North Carolina on Wednesday, the crowd broke out into chants of "Send her back!"

Trump later said he was not happy with the chant but has since characterized the crowd as "incredible patriots."

Earlier last week, the Democratic-led House passed a resolution condemning his tweets directed at the four lawmakers.