U.S. State Department spokeswoman Jen Psaki said officials are investigating whether the content of the video is real. If it is, Ms. Psaki said, "we are sickened."
In the video, reporter Steven Sotloff appears in an orange jumpsuit near a man in black with his face hidden. Mr. Sotloff was on his knees in a desert area similar to that shown in the recent video of the execution of U.S. reporter Jim Foley.
The militant in this latest video made a threat against a British hostage he called David Haines. The militant warned governments to back off "this evil alliance of America against the Islamic State.”
It was unclear when the video was shot.
Mr. Sotloff was kidnapped in Syria in August 2013. He was working for Time and Foreign Policy magazines at the time.
A spokesman for President Barack Obama’s administration said he could not confirm the execution. But he said "our thoughts and prayers" are with Mr. Sotloff’s family.
Pakistani Parliament Support PM Sharif
Pakistan’s parliament opened an emergency meeting Tuesday. It expressed political support for Prime Minister Nawaz Sharif. Thousands of protesters opposed to his rule gathered outside the building at the same time.
The protesters have repeatedly demanded the prime minister’s resignation and a new election. They say Mr. Sharif’s party won the election by cheating.
Three weeks of anti-government protest in Islamabad have led to fears of a military overthrow in the country.
Did US Drone Strike Kill al-Shabab Leader?
Officials are trying to learn whether a U.S. drone attack in Somalia killed Ahmed Abdi Godane, the leader of militant group al-Shabab.
Somali officials said Monday's attack targeted top al-Shabab leaders meeting in southern Somalia. Witnesses said the drone targeted three vehicles traveling on a road. They say several missiles hit two of the vehicles but the third may have escaped.
Ahmed Abdi Godane is 37 years old. The U.S. State Department has named him one of the most wanted terrorists in the world.
NATO to Meet on Russia and Middle East
The leaders of the 28 NATO member countries will gather for the first time in two years starting Thursday in the Welsh city of Cardiff.
Delegates are to talk about the possible establishment of a force that could deploy quickly in reaction to threats in eastern Europe. That discussion may center on the conflict between Ukraine and pro-Russian separatists.
NATO leaders will also discuss the threat from the Islamic State militant group that has captured large parts of Iraq and Syria. The U.S. has carried out air strikes targeting the group. And the U.S. and other NATO members have been providing humanitarian aid.