A Syrian cinematographer who worked on an Oscar-nominated documentary has been barred from entering the United States to attend Sunday's Academy Awards ceremony.

Despite being granted a US visa, Khaled Khatib, 21 - who documented Syria's civil war in the film The White Helmets - was scheduled to depart from the Turkish capital Istanbul on Saturday before US officials reportedly discovered "derogatory information" about him.

The US Department of Homeland Security at the last minute blocked Khatib from travelling to Los Angeles, saying he now needed a new passport waiver, according to documents seen by The Associated Press news agency.

Derogatory information is a broad term that includes a variety of allegations, ranging from connections to attacks to passport irregularities.

According to the Department of Homeland Security correspondence, Khatib was detained by Turkish authorities in Istanbul last week, which led to the US barring him.

"A valid travel document is required for travel to the United States," Department of Homeland Security spokeswoman Gillian Christensen told AP.

It was unclear why he was arrested by Turkish authorities, and his current whereabouts are unknown.

Calls and emails to Khatib by Al Jazeera were not answered.

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The incident took place less than a month following President Donald Trump's now suspended temporary travel ban, which restricted entry of people from seven Muslim-majority countries, including Syria.

Khatib along with fellow rescue worker Raed al-Saleh were both granted visas earlier this month to attend the Oscars on Sunday.

In a Twitter post, Saleh said on Friday they would not be attending the event because they are busy helping people caught in the conflict.

Regretfully we won't be at the OSCARS due to intensity of work, our priority continues to be helping civilians & rescue operations in #Syria — Raed Al Saleh (@RaedAlSaleh3) February 24, 2017

The White Helmets is a 40-minute Netflix documentary nominated in the Oscars' short documentary category. It gives a glimpse of the daily lives of the group - also known as Syria Civil Defence - whose members volunteer as emergency rescue workers in rebel-held parts of Syria.

Among the film's harrowing scenes are images of people digging through demolished homes searching for potential survivors after air strikes or artillery barrages.

Volunteer medical workers are at risk of being killed in "double tap" attacks that target first-responders arriving at the scene and many have died.

The group was also nominated for a Nobel Peace Prize last year.

Since the start of the Syrian war that erupted in March 2011, the United Nations estimates nearly 400,000 people have been killed and more than half the population has been forced to flee the country.