Hussein Malla, Associated Press
FILE - In this March 28, 2018 file photo, a Syrian fighter from the Kurdish police force, guards the entrance of the Manbij Military Council, the Kurdish-led, US-backed, militia group that defends Manbij, Syria. Early Tuesday, March 26, 2019, gunmen attacked a checkpoint at one the entrances of Manbij, killing seven fighters, an official with the council said. The Britain-based Syrian Observatory for Human Rights, a war monitor, said sleepers cells of the Islamic State group carried out the attack.

BEIRUT — The Islamic State group claimed responsibility for an attack on a checkpoint of U.S.-backed forces in the northern Syrian town on Manbij that killed seven fighters on Tuesday.

The attack indicates that IS can still launch deadly insurgent strikes and is far from being completely defeated, despite losing all the territory it once controlled in Iraq and Syria. Islamic State militants were blamed for several attacks in recent months in and around Manbij, which was declared liberated in 2016.

Sharfan Darwish of the Manbij Military Council told The Associated Press that the attack came shortly after midnight at one of the entrances to Manbij, targeting the fighters who "were carrying out their mission of protecting" the town.

The Britain-based Syrian Observatory for Human Rights, a war monitoring group, said an IS sleeper cell carried out the attack, and that three other Manbij Military Council fighters were wounded.

In its claim of responsibility, IS said its followers targeted a checkpoint on the western edge of Manbij and confiscated the weapons of the U.S.-backed fighters.

In January, IS claimed a suicide attack in Manbij that killed 19 people, including two U.S. soldiers and two American civilians.

The fate of Manbij has been a source of tension between the U.S. and Turkey, which says the Manbij Military Council is linked to its Kurdish insurgency.

Turkey insists on the withdrawal of the Syrian Kurdish-led militia, which liberated Manbij from the Islamic State group in 2016.

In a separate development, Russia and Syria urged the U.S. to discuss the evacuation of a refugee camp in southern Syria.

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The Russian and Syrian refugee return coordination centers said the U.S. ignored their invitation to attend Tuesday's meeting on the closure of the Rukban camp near the Jordanian border. They said in a statement that the meeting included tribal leaders from the camp and U.N. officials.

Russia and Syria say the camp's residents are suffering from malnutrition and disease and should be allowed to leave for government-controlled areas. They criticized the U.S. for taking an "unconstructive" stance and denying entry to the camp. The U.S. says it has allowed aid delivery to Rukban.