The foreign ministry in Turkmenistan accused the government of Azerbaijan, in a letter, of "unlawful actions" at a disputed offshore field in the Caspian Sea, the Russian news service RIA Novosti reports.
Turkmenistan and Azerbaijan haven't agreed on maritime borders since gaining independence from the Soviet Union in 1991.
Workers from Azerbaijan carried out an exploration program in the area in the 1980s.
Azeri authorities had reminded Turkmenistan that a 2008 agreement suspends exploration or development in the region until both sides determine maritime borders.
The field, called Kapaz on the Azeri side and Serdar in Turkmenistan, holds an estimated 580 million barrels of oil and 1.1 trillion cubic feet of natural gas.
Iran, meanwhile, said it discovered oil in the Caspian Sea in waters that might be claimed by Azerbaijan, the semiofficial Fars News Agency reports.