Pakistan blacklisted and denied entry to the Asia coordinator of the Committee to Protect Journalists, the global press freedom group's executive director said on October 18.
Steven Butler was refused entry at the Allama Iqbal International Airport in Lahore despite having a valid visa and was returned to the United States.
Butler said he was told he was on "a stop list of the Interior Ministry."
Joel Simon, CPJ's executive director, called the expulsion of Steven Butler "baffling" and "a slap in the face" to those concerned about press freedom in Pakistan.
"Airport authorities in Lahore confiscated Butler's passport and forced him onto a flight to Doha, Qatar," Simon said in a statement.
The Pakistani government had no immediate comment on October 18.
"Pakistani authorities should give a full explanation of their decision to bar Butler from entering and correct this error," Simon said.
If the government is interested in demonstrating its commitment to a free press, it should conduct a swift and transparent investigation into this case," he added.
Butler was planning to attend the Asma Jahangir Conference-Roadmap for Human Rights in Pakistan. The event scheduled for this weekend is named for a renowned Pakistani human rights activist who died last year of a heart attack.
Butler had been a regular visitor to Pakistan, working with journalists' groups throughout the country.
"Pakistani authorities’ move to block Steven Butler from entering the country is baffling and is a slap in the face to those concerned about press freedom in the country," Simon said.
Pakistani journalists have been subjected to increasing censorship in the past year.
Websites have been shut down, including the Urdu website of the U.S. government-funded Voice of America, after it reported on an ethnic Pashtun tribal movement that's been critical of military operations in regions bordering Afghanistan.
RFE/RL's Radio Mashaal's bureau has also been shut down.
Leading newspapers, including the well-known English-language Dawn Newspaper, and their distributors have also come under pressure.
Journalists and media freedom groups blame the powerful Pakistani military, which they say are seeking to stifle criticism as well as any coverage of the Pashtun movement against the military's war on terrorism in the tribal regions.
The military denies the accusations.
With reporting by AP