More than 60 Iranians and Iranian-Americans have been subject to questioning and lengthy delays while crossing back into the US from Canada in Blaine, Washington state, according to a US Muslim rights group.

The news comes amid heightened tensions between Washington and Iran, after the killing of Qassem Suleimani, Iran’s top general, in a drone strike ordered by Donald Trump.

The Council on American-Islamic Relations (Cair) in Seattle said many more Iranians were refused entry to the US due to a lack of capacity for Customs and Border Patrol (CBP) to detain them as they returned from an Iranian pop concert in Vancouver, Canada.

Cair, a civil liberties and advocacy organization, said many had passports confiscated and were questioned about their political views and allegiances.

“These reports are extremely troubling and potentially constitute illegal detentions of United States citizens,” Masih Fouladi, executive director of Cair-WA, said in a statement.

Fouladi said the group was “working to verify reports of a broad nationwide directive to detain Iranian-Americans at ports of entry so that we can provide community members with accurate travel guidance”.

One of those detained, according to Cair, was a 24-year-old US citizen and medical student who was allegedly held and questioned for more than 10 hours with her family.

“The vast majority of people being held last night were American citizens,” said the student, who gave her name as Crystal. “We kept asking why we were being detained and asked questions that had nothing to do with our reason for traveling and was told ‘I’m sorry this is just the wrong time for you guys.’”

Michael Friel, a CBP spokesman, disputed the account, as well as an allegation that officials had been ordered to hold travelers with an Iranian background, regardless of their immigration status.

“Social media posts that CBP is detaining Iranian-Americans and refusing their entry into the US because of their country of origin are false. Reports that DHS/CBP has issued a related directive are also false,” he said.

The Cair statement included a claim that an order had been issued to border agents to “report” and detain anyone of Iranian heritage, regardless of citizenship status, who is deemed potentially suspicious or “adversarial”.

On Friday, federal security agencies and law enforcement departments in several major US cities issued alerts advising residents to be vigilant after the US strike against Iran’s top general. Officials said they were stepping up security efforts at sensitive locations.

Chad Wolf, acting homeland security secretary, said his department was working with law enforcement and other groups across the US and was ready to respond to any threats though none had been detected.

In New York, Mayor Bill de Blasio announced he was consulting with city officials about steps the the police will take to protect key locations “from any attempt by Iran or its terrorist allies to retaliate against America”.

The New York police commissioner, Dermot Shea, added that while “there are no specific [or] credible threats to [New York], we’ve deployed additional resources to key locations. As always, if you see something, say something.”

Los Angeles, home to a sizable US-Iranian population, also issued an alert hours after the killing of Qassem Suleimani, asking “every Angeleno to say something if you see something”.