"The United States embassy in Pakistan calls for the immediate release of a US diplomat unlawfully detained by authorities in Lahore," said a statement issued by the American mission.
The arrest and remanding into custody of the diplomat violated "international norms and the Vienna Convention," the statement said.The US national -- identified by media as Raymond Davis -- has contended that he acted in self-defence after two youths allegedly tried to rob him in a congested area of Lahore on Thursday.
A third Pakistani was killed when he was hit by a US consulate vehicle rushing to the aid of the American.
Pakistani officials have said the American has been booked for murder and would be prosecuted according to domestic laws.
Though an earlier statement issued by the US Embassy had described the American as a "staff member of the US Consulate General" in Lahore, today's statement said he was a diplomat assigned to the mission in Islamabad.
He has "a US diplomatic passport and Pakistani visa valid until June 2012," the statement said.
It further said that "the diplomat acted in self-defence when confronted by two armed men on motorcycles" on Thursday.
"The diplomat had every reason to believe that the armed men meant him bodily harm. Minutes earlier, the two men, who had criminal backgrounds, had robbed money and valuables at gunpoint from a Pakistani citizen in the same area," the statement said.
After being detained, the US diplomat "identified himself to police as a diplomat and repeatedly requested immunity under the Vienna Convention on Diplomatic Relations."
The statement said: "Local police and senior authorities failed to observe their legal obligation to verify his status with either the US Consulate General in Lahore or the US embassy in Islamabad.
"Furthermore, the diplomat was formally arrested and remanded into custody, which is a violation of international norms and the Vienna Convention, to which Pakistan is a signatory.The US embassy said it regretted that Thursday's incident "resulted in loss of life."
It said: "We greatly value the cooperation and partnership between Pakistan and the United States, which is vital to the interests of both countries.
"The US embassy is committed to working closely with the Pakistani government to secure the immediate release of the diplomat, as required under Pakistani and international law."
The US has launched a flurry of diplomatic activity in Washington and Islamabad to seek diplomatic immunity for the American.
The US deputy chief of mission and another senior diplomat separately met officials of the Foreign Office's protocol division, which handles issues related to diplomatic immunities and privileges.
US Ambassador Cameron Munter met Interior Minister Rehman Malik and Foreign Secretary Salman Bashir.The US State Department was also working with the Pakistani embassy in Washington on the issue.
Interior Minister Malik also met army chief Gen Ashfaq Parvez Kayani yesterday reportedly to discuss the issue