"We are very disappointed by Laurent's actions, which are against our clear instructions," Arsenal said in a statement.
Thursday 11 July 2019 18:50, UK
Arsenal captain Laurent Koscielny has refused to travel with the squad on their pre-season tour to the United States, the club have announced.
The Frenchman was expected to be part of the squad flying to the USA on Thursday but the club have now said the 33-year-old will stay behind.
Sky Sports News understands Koscielny wants to return to France and has asked Arsenal to release him from his contract, which has one year remaining.
Arsenal said in a statement: "Laurent Koscielny has refused to travel to the US for our pre-season tour.
"We are very disappointed by Laurent's actions, which are against our clear instructions.
"We hope to resolve this matter and will not be providing any further comment at this time."
Koscielny remains an important player for Unai Emery. He appeared in 29 games for Arsenal in all competitions last season, starting 24.
In total, the France international has made 255 appearances for Arsenal in the Premier League, scoring 22 goals since joining from French side Lorient for £10m in July 2010.
Arsenal, meanwhile, are close to completing a deal to sign 18-year-old centre-half William Saliba from Saint Etienne, Sky Sports News understands.
However, that deal is expected to see Saliba return on loan to the French club next season, meaning he would not bolster Emery's defensive options for the upcoming 2019/20 campaign.
Arsenal are due to face Bayern Munich in Carson, California on July 18 in the opening game of their pre-season tour.
That is followed by games against Fiorentina in Charlotte on July 20 and Real Madrid on July 24 in Washington DC.
Sky Sports' Nick Wright: The situation leaves Arsenal's captaincy shrouded in uncertainty. If Koscielny is stripped of the armband, or if he gets his wish and leaves the club, then Emery's options are limited.
The Spaniard named a captaincy group of five players last season, Koscielny being one of them, but of the other four, Petr Cech and Aaron Ramsey are gone and Mesut Ozil's future is uncertain. That leaves Granit Xhaka as the only logical option, although the Swiss international is not universally popular among supporters.
Could Emery look elsewhere? There are few standout candidates. Nacho Monreal has the experience having been at the club since 2013, but his place in the team is far from guaranteed. Sokratis could be seen as captaincy material, but he has only been at the club for a single season. Hector Bellerin, an Arsenal player since 2011, might be a left field alternative.
It is another conundrum for Emery in what's turning into a difficult summer.