An Islamic State (ISIS/ISIL) suicide bomber targeted a joint convoy of U.S. troops and Kurdish-led Syrian Democratic Forces (SDF) in northern Syria on Monday, marking the second attack against American service members in less than a week and underscoring the lingering menace posed by the jihadi group.

According to the U.K.-based Syrian Observatory for Human Rights (SOHR), a group that uses ground sources to monitor the conflict, Monday’s attack reportedly killed five fighters from the SDF, an Arab-Kurdish alliance primarily consisting of fighters from the Syrian Kurdish People’s Protection Units (YPG/YPJ).

ISIS also injured two Kurdish fighters, the Associated Press (AP) reports, citing the northern Syria-based Kurdish Hawar news agency.

The attack came days after a suicide attack claimed by ISIS killed 19 people, including four Americans, in northern Syria’s Manbij region on January 16 and wounded others including three U.S. service members, reportedly marking the deadliest assault on United States forces since they deployed on the ground in Syria in 2015.

Monday’s assault reportedly took place on a checkpoint on the edge of the town of Shaddadeh in the Syrian province of Hassakeh. The checkpoint sits on a road used by local U.S.-allied Kurdish troops and the American-led coalition fighting ISIS militants near the Iraqi border.

U.S. military Col. Sean Ryan revealed that the American-led coalition did not suffer any casualties, AP notes. However, Turkey’s state-run Anadolu Agency (AA), citing local sources, reports that the ISIS suicide bomber seriously wounded an American service member. The SOHR also reports that the attack injured two American service members. Pentagon officials seldom report details of U.S. troops wounded in battle.

“We can confirm a combined U.S. and Syrian partner force convoy was involved” in the suicide bomb attack, he said, adding, “We will continue to review the situation and provide updates as appropriate.”

ISIS claimed responsibility for both of the recent attacks via its Amaq news agency, AP notes.

U.S.-led coalition forces and local fighters have decimated ISIS’s territorial caliphate in Iraq and Syria, but the group still controls about one percent of the territory it once held — located in northern Syrian near the Iraqi border — from where it is still able to carry out deadly attacks.

The recent ISIS attacks are a testament to independent and U.S. government assessments that the jihadi group remains a threat.

Echoing non-government experts, the Pentagon’s inspector general reported last November that the jihadi group has morphed into “an effective clandestine ISIS organization” that “has moved underground and is acting as an insurgency” still in both Iraq and Syria even though it only controls one percent of the territory it once held.

The Kurdish People’s Protection Units (YPG) lead and make up the majority of the U.S.-backed SDF, a group fighting ISIS in northern Syria. YPG fighters are the armed wing of the Democratic Union Party (PYD) that has maintained control of large swathes of Syria with the help of the United States.

Although Turkey considers the YPG and PYD to be affiliated with the Kurdistan Worker’s Party (PKK) communist terrorist group, the United States continues to lend support to the Kurdish fighters, arguing that they are the most effective force against ISIS.

After announcing plans to withdraw the estimated 2,000 American troops from Syria, U.S. President Donald Trump signaled plans to slow down the process.

In the wake of criticism from U.S.-allied Kurds who accused the U.S. of abandoning them after the withdrawal announcement, Trump’s National Security Advisor John Bolton conditioned the pull out on assurances from Turkey that the Kurds will be safe and ensuring ISIS’s lasting defeat.

U.S.-backed Kurds warned against a precipitous American military withdrawal, saying the move would render them vulnerable to attacks by Turkey and could lead to an ISIS revival.

SOHR now reports that the U.S. troops “are packing and preparing to withdraw earlier than the time spoken about” in light of U.S.-Turkey talks of establishing a safe zone in northern Syria, noting that the American service members are making preparations to leave Syria “as the last confrontation are underway to end the ‘Islamic State’ organization” in the organization’s last enclave, east of Euphrates river.

Bolton said a residual American force would remain in southern Syria to combat the Iranian threat. SOHR notes that the U.S.-allied Kurds are turning to the Russian and Iranian-backed regime of Bashar al-Assad for assistance against an imminent attack from Turkey.