The United States demanded the immediate release of an American diplomat arrested in the shooting deaths of two Pakistan men, saying Saturday that the man had immunity from prosecution and was illegally detained.

The statement from the embassy raised the stakes in what could emerge as a major dispute between Pakistan and the United States, which is pushing for closer ties with Islamabad to help fight Islamist militancy.

The embassy said the man was acting in self-defense on Thursday in Lahore when he shot and killed two armed men as they approached his car. It said the men were intending to rob him, something police officers have also said.

The man, who has not been named, was taken into custody soon after the shooting and police have said they are investigating him and could file murder charges. U.S. officials were only granted access to him late Friday.

"The United States Embassy in Pakistan calls for the immediate release" of the diplomat, the statement said.

Pakistani officials were not immediately available for comment. They have largely ignored questions over any immunity the American may have.