The U.S. has bolstered defenses at a base in Jordan that was attacked by Iran-backed militants as it prepares for a wider U.S. response to a drone attack that killed three service members, a U.S. official said Friday.

Even as a larger U.S. military response seemed imminent, some Iran-backed factions pledged to continue to attack U.S. forces in the region. In a statement released Friday, one of Iraq’s strongest Iran-backed militias, Harakat al-Nujaba, announced its plans to continue military operations against U.S. troops, despite other allied factions having called off their attacks in the wake of a Sunday drone strike that killed three U.S. service members in Jordan.

While previous U.S. responses have been more limited, the attack on Tower 22, as the Jordan outpost is known, and the deaths of the three service members has crossed a line, the official said. The base was struck by an Iranian-made drone fired from Iraq, the official said.

In the days since the attack, the U.S. has bolstered the defenses around Tower 22, which houses about 350 U.S. troops and sits near the demilitarized zone on the border between Jordan and Syria. The Iraqi border is only 6 miles (10 kilometers) away.

In response, the U.S. is weighing response options to include striking militia leaders. The U.S. options under consideration include targets in Syria, Yemen and Iraq, where the Iranian-made drone that killed the service members was fired from, the official said. The U.S. has blamed the Jordan attack on the Islamic Resistance in Iraq, a coalition of Iranian-backed militias.

On Thursday Defense Secretary Lloyd Austin indicated that the U.S. response against the militias would widen.

“At this point, it’s time to take away even more capability than we’ve taken in the past,” Austin said in his first press conference since he was hospitalized on Jan. 1 due to complications from prostate cancer treatment.

Austin said that Iran has had a hand in the attacks by supplying and training the militias. The U.S. has tried to communicate through backchannels to Iran over the last few months to get them to rein in the militant groups, a second U.S. official said.

The officials spoke on the condition of anonymity to discuss details that have not been acknowledged publicly.

The U.S. has also tried more limited military responses in a series of strikes against weapons storage sites and training areas. So far, the U.S. response has not deterred the groups, which have attacked U.S. facilities at least 166 times since October.

Kataib Hezbollah, another powerful Iranian-backed Iraqi militia, which has been watched closely by U.S. officials, said Tuesday it would “suspend military and security operations against the occupying forces” to avoid embarrassing the Iraqi government.

The attack on Tower 22 led to the first deaths of U.S. service members since the war between Israel and Hamas broke out and President Joe Biden, Austin and Chairman of the Joint Chiefs Gen. CQ Brown were traveling to Dover Air Force Base in Delaware on Friday to be with the families of the fallen as the soldiers’ remains are honored at a dignified transfer ceremony.