Noureddine Chouchane, a senior operative in the terrorist group from Tunisia, was believed to be among those from around Africa and the Middle East who had converged on the site, Earnest said. It was not immediately clear whether Chouchane was killed, Earnest said.
Chouchane is thought to have played an instrumental role in two terrorist attacks in Tunisia
last year, one at Tunis' Bardo Museum
that killed 23 people and another at a seaside resort in Sousse
that left 38 people dead. ISIS claimed responsibility for both massacres.
Friday morning's U.S. strike in the al-Qasser district in Sabratha, a coastal city in northwestern Libya where most residents are from Tunisia, killed at least 49 people, Hussain al-Thawadi, the Mayor of Sabratah, said.
Al-Thawadi told Libya TV in an interview the death toll could rise because more people might still be under the rubble. He said the house was rented by suspected ISIS members and he believed more than 60 people were inside it when it was hit.
Also six people were wounded, according to the Sabratha Municipal Council.
A Libyan man started to expand the house in Sabratha to several levels last year, security officials in the city said. He had brought in several groups of fighters over the past few months, including one batch two days ago. That house was struck Friday.
Over the last several weeks, the United States observed militants moving around the site and undergoing what appeared to be special training, according to a U.S. official.
"This was outside the normal training camp scenario," the official said.
The activity raised concerns the people there might be planning to launch an external attack, though no details were discovered about where or when this might take place.
Al-Thawadi described the site as an ISIS recruitment base for foreigners, mostly from Tunisia. He said a Jordanian woman was among the dead.
The U.S. military has launched hundreds of airstrikes against ISIS targets over the past two years. These have been concentrated in Iraq and Syria
, where the Islamist extremist group has established its biggest foothold and has its de facto capital in Raqqa.
-- a North African nation that's been in turmoil, and a hotbed for some militant groups, since a 2011 revolution that toppled its longtime dictator Moammar Gadhafi
-- has been in its crosshairs as well.
ISIS expansion in turbulent Libya
ISIS has emerged as the world's top terrorist threat, having conducted or inspired about 70 attacks in 20 countries since declaring its caliphate in June 2014. Not including its armed campaigns in Syria and Iraq
, these attacks have killed at least 1,200 people and injured more than 1,700 others.
It is significant that ISIS leader Abu Bakr al-Baghdadi exerts more control over the ISIS branch in Libya than any other, according to a report late last year to the United Nations Security Council
. This conclusion is in line with U.S. intelligence determinations that al-Baghdadi sees the relatively lawless, impoverished North African state as prime ground to enlarge his caliphate.
As such, ISIS has been expanding into
in many places in Libya, like Gadhafi's hometown of Sirte. The group has asserted itself by taking over territory and sadistically exercising terror, as evidenced by its beheadings of Egyptian Coptic Christians
about a year ago on a Libyan beach.
Libya has also been a base to train militants, devise plots and launch them in places like neighboring Tunisia, which has been considered the Arab Spring
's success story but has not been immune to the violence wrenching the region.
The Bardo Museum and Sousse beach attacks are gut-wrenching proof of that, not just because of the human carnage but also for their negative effects on a Tunisian economy that's long benefited from tourism.
Both attacks were carried out by one terror cell whose members came from Tunisia, communicated with ISIS leaders in Syria and Iraq, and trained in Libya, a Western counterterrorism source said. That training happened near Sabratha, about 60 miles (100 kilometers) east of the Tunisian border where the Friday morning strike took place.
The same cell planned to attack France's diplomatic complex in Tunis, only to be thwarted as Tunisian security forces moved in, according to the source. All of its roughly half dozen core members -- at least those not killed in attacks -- who were still in Tunisia are now in custody. But others involved in the plots may not be in the country, possibly finding refuge in their former training ground in Libya instead.
U.S. official called for 'decisive military action' in Libya
"I emphasized to the (Tunisian) President that the United States is fully committed to working with Tunisia so that it can continue to build on (its past) success," Obama said
in Washington alongside his Tunisian counterpart, Beji Caid Essebsi.
Chouchane has been a focus for Tunisia since its Interior Ministry identified him and several others as planners of the Bardo Museum attack.
The 30-something man hails from the central town of Chouachnia in the Sidi Bouzid governorate, an area that's a breeding ground for many jihadists.
Friday's airstrike isn't just about Chouchane or Tunisia. It's also about the larger fight against terrorists, particularly in Libya.
A U.S. airstrike in Libya last November killed Abu Nabil
, an Iraqi national and longtime al Qaeda operative who'd become an ISIS leader, the Pentagon said.
Less than a month later, the American military killed Abdirahman Sandhere, a senior leader of the al Qaeda-affiliated group Al-Shabaab
, according to a statement from Pentagon spokesman Peter Cook. That was around the time U.S. Special Operations forces reportedly landed in Libya, only to turn around and head back a short time later.
And a few weeks ago, Obama's top military adviser talked about stepping up efforts to curtail ISIS specifically in the North African country.
Addressing reporters while traveling in Europe, Gen. Joseph Dunford, the Joint Chiefs of Staff chairman, said the United States wants to "take decisive military action" to "check" ISIS in Libya.