A United Airlines flight from San Francisco to Shanghai, China was diverted from flying over Russian airspace for “diplomatic reasons” early Wednesday morning, passengers on the flight say.

Those diplomatic concerns forced the flight to refuel at Tokyo’s Narita airport, the flight’s captain told passengers, several of whom posted about it on social media.

On @United 857 enroute to PVG, diverted to NRT. Captain: Due to diplomatic reasons, Russia denied access to air space. Now refuelling — brianlinca (@brianlinca) April 12, 2017

One passenger named Andy Brown says that the airplane almost ran out of fuel before landing at Narita.

“Then they backed into something at Tokyo airport broke something on the plane!” he wrote in a Facebook post.

“United probably will not tell the public about this.”

Reached by Skype, Brown told The Daily Caller that he makes the flight to Shanghai about once a month and has never had a fight diverted.

Another passenger posted about the delay.

теперь шутки про третью мировую не кажутся такими смешными… pic.twitter.com/4FDcsjSitp — латышева (@latyshevaaa) April 12, 2017

@latyshevaaa @etosova Morning @united !

Can you clarify why flights #UA857 and #UA198 were diverted to NRT&HND from their destination in Shanghai, please? — Rǫgnvaldr ór Moskviæ (@1stChairmanMeow) April 12, 2017

It is not entirely clear what diplomatic concerns may have forced the diversion. The most obvious explanation would be the tense relationship between the U.S. and Russia over Syria’s use of chemical weapons during an attack on April 4.

The White House said on Tuesday that the Russian government has engaged in a cover-up to shift blame from the Syrian regime for the attacks, which involved the use of sarin gas and killed 100 people. Syria and Russia are allies.

In 2014, Russian Prime Minister Dmitry Medvedev floated the idea of denying Western airlines access to Russian airspace in response to sanctions.

Secretary of State Rex Tillerson is also currently in Russia. He met with Russian Foreign Minister Sergey Lavrov on Tuesday and will meet with Vladimir Putin on Wednesday.

It is unclear if any other flights were re-routed and whether the diversions are temporary.

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Flight logs tracked on the website FlightAware show that the flight from San Francisco was diverted at 7:32 a.m. Eastern Standard Time. The flight, which normally takes less than 13 hours, was delayed nearly three hours. It left San Francisco’s airport at 2:10 p.m. local time on Tuesday and arrived in Shanghai at 8:22 p.m. local time.

A spokesman for United Airlines confirmed to The Daily Caller that the flight was re-routed and that because the airplane “required a longer route,” it was forced to refuel. The spokesman declined to comment on whether diplomatic concerns were at play. He said that United is in communication with air traffic control, including the Federal Aviation Administration, to determine the reason for the diversion.

A spokesman for the Federal Aviation Administration sent this statement after publication:

“A U.S. flight was routed around Russian airspace Tuesday. These kinds of reroutes are routine and most often occur when there are issues with the paperwork that is filed to obtain clearances to access that airspace.”

This post has been updated with additional information.

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