A BP subsidiary working in the Caspian Sea announced it reached a landmark for oil pulled from Azeri waters with its 3 billionth barrel.

BP's regional office for Azerbaijan, Georgia and Turkey began work at the supergiant Azeri-Chirag-Gunashli oil field in the Caspian Sea in 1997 and, more recently, started operations at the West Chiraq platform tied to the field.

Gordon Birrell, the regional president for BP, said in a statement reaching 3 billion barrels of oil produced from the field "is a significant milestone for both BP and for Azerbaijan."

Four years ago, the government in Azerbaijan criticized BP's role in the ACG complex, complaining production during the first half of 2012 was down 12 percent from the previous year. Azerbaijan's President Ilham Aliyev said at the time that the situation at the field was "unacceptable."

States sharing borders to the Caspian Sea are at odds over territorial boundaries. The Iranian government once threatened to sue BP for pollution in the Caspian Sea. Tehran's environmental agency said BP dumped oil waste into the Caspian Sea after exploration campaigns in Azeri waters.

ACG, operated by BP and co-owned by the Azeri government, is one of the largest oil complexes in the world. BP also manages the Azeri offshore Shah Deniz natural gas field, seen as a major potential source for European energy consumers.

One of the oldest oil-producing nations in the world, Azerbaijan is estimated to hold as much as 7 billion barrels of oil reserves. Production last year averaged around 850,000 barrels of oil per day.