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Nigeria has advised against non-essential travel to the United States after some citizens had their visas canceled and were denied entry.

In one case, a doctor was refused entry at Hartsfield-Jackson Atlanta International Airport. The physician, who was due to attend a wedding, was left traumatized, according to Abike Dabiri-Erewa, a senior special assistant to Nigeria's President on foreign affairs and diaspora.

She told CNN: "Four Nigerians have been sent back in the past two weeks so we are advising that those without any urgent need to travel should consider rescheduling their trip until there is clarity on the new immigration policy."

Dabiri- Erewa added that "no reasons were given for the decision by the US immigration authorities."

"Then there was the case of the engineer who was asked to prove he was an IT expert, " Dabiri-Erewa said, referring to software engineer Celestine Omin, 28, who was detained at John F. Kennedy International Airport in New York. He was on a work trip from Lagos to his employer, startup Andela.

According to Andela, Omin was told he didn't look like an engineer and needed to take a test to prove it. He was then asked to answer two difficult computer science questions. Omin was later released after three hours following a call between US Customs and Border Protection and his employer.

"It is advice that we feel we have to issue," Dabiri-Erewa said. "We have to inform the people about what's going on."

Nigeria is not on the list of countries that face a travel ban to the United States under President Donald Trump's executive order exempting immigration from certain countries.

Trump signed a new executive order Monday that bans immigration from six Muslim-majority countries, dropping Iraq from a previous order in January.

The new travel ban comes six weeks after Trump's original order caused chaos at airports nationwide before federal courts blocked it.