WASHINGTON — U.S. officials say they have seen no evidence yet to suggest that Friday’s series of deadly attacks across three continents were linked, but they are continuing to investigate.

“At this point it’s too soon to tell whether or not these various and far-flung attacks were coordinated centrally or whether or not they were coincidental,” said Army Col. Steve Warren, a Pentagon spokesman. “We are continuing to look into this.”

In Tunisia, a gunman killed 37 people, mostly tourists, at a seaside resort. In Kuwait, an Islamic State suicide bomber killed at least 27 people at a Shiite mosque. In France, a driver rammed his truck into a U.S.-owned gas factory, where a decapitated body was found with the head hanging at the entrance. In all, at least 65 people were killed across the three continents.

The attacks may not have been centrally directed but were likely loosely coordinated among extremist groups, such as al-Qaeda and the Islamic State, which share similar goals of attacking Western and Shiite Muslim targets, analysts said.

The groups may have been planning attacks separately and agreed among themselves to launch them on the same day, said Rick Nelson, an analyst at the Center for Strategic and International Studies.

“That’s just organizations with a common goal loosely operating together to achieve a greater effect,” Nelson said. “That’s what you have here.”

The attacks come as the Islamic State, also known as ISIL or ISIS, has grown in prominence and has begun to overshadow al-Qaeda, which it broke away from. The Islamic State has encouraged followers to create their own attacks.

“We do know that ISIL encourages lone-wolf attacks,” said Col. Patrick Ryder, a spokesman for U.S. Central Command, which oversees operations in the Middle East.

A number of extremist groups around the world have rebranded themselves under the Islamic State banner because they see the militants as overtaking al-Qaeda as the premier global terror organization.

In a video released Tuesday, the Islamic State called on its supporters to increase attacks during Ramadan, the Muslim holy month, and “be keen on waging invasion in this eminent month and commit martyrdom,” the Associated Press reported.

Homeland Security Secretary Jeh Johnson encouraged Americans to not be intimidated by the attacks. “We continue to encourage all Americans to attend public events and celebrate this country during this summer season, but always remain vigilant,” he said in a statement.

President Obama was briefed on the events throughout the day, White House Deputy Press Secretary Eric Schultz said.

“Obviously these are unfolding circumstances happening in real time,” Schultz told reporters aboard Air Force One while heading to Charleston, S.C., where Obama delivered a eulogy for Clementa Pinckney, the pastor and state senator killed in the June 17 church shooting there that left nine people dead.

Schultz said National Security Council staff members are “in touch with their counterparts in the respective countries and continue to offer help and assistance.”

The White House issued a statement, saying, “The United States condemns in the strongest terms the terrorist attacks in France, Kuwait, and Tunisia today. … As the president has discussed with his French, Kuwaiti and Tunisian counterparts in recent weeks, we are resolute and united in our shared effort to fight the scourge of terrorism.”

Contributing: Gregory Korte