Iranians have reported being harassed by US border officials amid diplomatic tensions following last week's US assassination of Iranian General Qasem Soleimani.
Several travellers of Iranian heritage told BBC News they faced inappropriate questions about their views.
One group said they were stopped for hours at the Canadian-US border at the weekend.
Democratic US lawmakers condemned the alleged incidents.
Around 60 Iranians and Iranian-Americans said they were stopped for up to seven-and-a-half hours on Sunday while trying to cross into the US at the Peace Arch Border Crossing near Blaine, Washington.
The Council on American-Islamic Relations (Cair), the largest Muslim advocacy group in the US, said the travellers had experienced "harassment" from US border officials.
Sepehr Ebrahimzadeh, a Seattle-based engineer, told BBC Persian he had waited about six hours to cross the border at Blaine, and was repeatedly questioned during that time.
A Canadian citizen with a US green card, he said he was trying to enter the US by land from British Columbia, Canada.
Mr Ebrahimzadeh said US Border Patrol guards had questioned him about his birthplace, his high school years in Iran, his own military service and his father's, and about other relatives and his employment history.
He said he saw other Iranians next to him who had to wait hours and were questioned about their social media accounts.
Cair said that some travellers were only allowed to proceed after 10 hours of questioning, while others were denied entry altogether.
Some, the advocacy group said, had their passports withheld while they were asked about their political views and allegiances.
University of Pennsylvania professor John Qazvinian said he was taken to a room and questioned "about the situation in Iran" upon landing at JFK's New York City airport on Sunday.
Well, just landed at JFK and — no surprise — got taken to the special side room and got asked (among other things) how I feel about the situation with Iran. I wanted to be like: my book comes out in September, preorder now on amazon— John Ghazvinian (@JohnGhazvinian) January 5, 2020
What are the authorities saying?
US Customs and Border Patrol (CBP) has issued a denial.
"Social media posts that CBP is detaining Iranian-Americans and refusing their entry into the US because of their country of origin are false," CBP spokesman Mike Friel said in a statement.
The agency said security measures at border patrol crossing points have been ramped up amid heightened tensions with Iran.
Average waiting times at the Blaine crossing point were around four hours when the Iranian group was attempting to cross, CBP added.
The agency said the longer wait was due to a higher volume of travellers during the holiday season.
Social media posts that CBP is detaining Iranian-Americans and refusing their entry into the U.S. because of their country of origin are false. Reports that DHS/CBP has issued a related directive are also false.— CBP (@CBP) January 5, 2020
What other reaction is there?
US lawmakers expressed concern on Twitter about the reports.
Senator Elizabeth Warren, a leading contender in the Democratic presidential race, called the reports "deeply disturbing".
"Iranian Americans have the same rights as all other U.S. citizens and should be treated with dignity and respect at our border - not bigoted, xenophobic scrutiny," Ms Warren said on Twitter.
These reports are deeply disturbing. Iranian Americans have the same rights as all other U.S. citizens and should be treated with dignity and respect at our border—not bigoted, xenophobic scrutiny. https://t.co/FOYVZSG0K5— Elizabeth Warren (@ewarren) January 6, 2020
Washington state congresswoman Pramila Jayapal also said she was "deeply disturbed".
California congresswoman Barbara Lee urged anyone with information to call her office about "the detention of Iranian nationals, including US citizens and green card holders by border guards".
People of Iranian descent — including American citizens & green card holders — are reportedly being detained by CBP officials. We can’t let this stand.— Rep. Barbara Lee (@RepBarbaraLee) January 5, 2020
If you or someone you know has been targeted or detained by CBP officials, please contact my district office at 510-763-0370.
Canada Border Services Agency said it "has no involvement in this matter".
"All Canadian citizens, regardless of their background, are equal before and under the law, and no one will ever be arbitrarily detained at the Canadian border nor refused entry purely because of their ethnicity or religion," they said in a statement provided to BBC News.