The United States says it is "deeply troubled" by the arrest of the head of an election watchdog in Azerbaijan.
In a statement, the U.S. ambassador to Baku, Richard Morningstar, urged the release of Anar Mammadli while his legal process continues.
Mammadli, the chairman of the Election Monitoring and Democracy Studies Center, was questioned by police in Baku on December 16 on charges including tax evasion and illegal business activity.
He has been ordered held for three months in pretrial detention.
Morningstar questioned whether Mammadli's detention was related to his organization's activities.
His center exposed widespread irregularities and fraud during elections this fall.
His arrest came a day before a visit to Baku by EU Energy Commissioner Gunther Oettinger and British Foreign Secretary William Hague.
The two attended the signing of multibillion-dollar gas pipeline deals on December 17.
The consortium developing Azerbaijan's offshore Shah Deniz-2 natural gas project signed a final investment agreement, paving the way for the first deliveries to Europe.
Azerbaijani President Ilham Aliyev said the agreements "will change the energy map of Europe."
British company BP is one of the main partners in the project.
The project will provide Europe with a new gas supply via a route that avoids Russian territory.
With reporting by Reuters