Islamic State militants have staged a surprise attack at a crossing frequently used by Iraqi and Syrian civilians seeking safety in north-eastern Syria, killing at least 37 people, mostly civilians, officials and activists said.
The pre-dawn attack took place after militants entered the village of Rajm Sleibi, on a front line that separates the Kurdish-controlled Hassakeh province from IS-held areas further south.
Some militants reportedly blew themselves up at a Kurdish checkpoint while others attacked sleeping civilians in a nearby temporary camp sheltering hundreds of displaced people who fled IS-controlled territory.
The International Rescue Committee said thousands of people from the Iraqi city of Mosul have travelled west to the Sleibi crossing since October, often through smugglers.
In a statement, it said several children were among the dead and wounded.
"We are appalled and saddened to hear of the attacks," said Thomas Garofalo, regional advocacy adviser at the IRC.
Redur Khalil, a spokesman for the main Kurdish fighting force in Syria, said the attack started with an early morning assault by IS militants on a checkpoint in Sleibi belonging to the Syrian Democratic Forces, a US-backed and Kurdish-dominated force that fights IS.
The militants then "committed a massacre" of civilians as they sought to enter SDF-controlled territory, Mr Khalil said.
He added that the attack came a few hours after IS suicide bombers dressed in civilian clothes entered the town of Shaddadeh and attacked SDF forces.
Issam Amin, a media activist in Hassakeh, said victims arriving at the city's hospitals had stab wounds.
The Britain-based Syrian Observatory for Human Rights, which tracks the Syrian conflict through activists on the ground, put the death toll at 38, including 23 civilians, many of them Iraqis.
IS is under attack by an array of forces in Syria and Iraq.
In Syria, the SDF is fighting to recapture the town of Tabqa, an important stronghold for the militants about 25 miles south east of their de facto capital, the city of Raqqa.
The SDF has pushed the extremists to northern neighbourhoods of Tabqa, close to one of Syria's largest dams, and Kurdish officials say the battle will be over soon.
In Iraq, the extremist group is fighting for survival against Iraqi forces and their allies in the last neighbourhoods it still holds in the western part of Mosul, Iraq's second largest city.
The group claimed Tuesday's attacks through its media arm, Aamaq, saying its fighters attacked four Kurdish positions in the southern countryside of Hassakeh province.
Rajm Sleibi is about 18 miles south of the town of al-Hol, which houses a large refugee camp for civilians displaced from Syria and Iraq. A Kurdish activist said it is the entry point to Hassakeh for Syrians civilians fleeing the eastern cities of Deir el-Zour and Raqqa, and those fleeing from Mosul and elsewhere in Iraq.
The civilians initially spend about two weeks in Rajm Sleibi while they get security clearance from Kurdish authorities, and from there are taken then to al-Hol.
The camp is within the zone of influence of the SDF but not immediately protected by its forces.
Hawar, a news agency for the semi-autonomous Kurdish areas in Syria, put the death toll at 37.
Elsewhere in Syria, opposition activists said IS killed 10 of its own after accusing them of selling weapons to arms traders.
The activists said the killings took place in the Mayadeen area in eastern Syria, near the Iraqi border.
The Observatory and Omar Abu Laila, a Europe-based activist with contacts in the area, said the men were all Iraqis and they were shot dead on Monday in a main street in front of dozens of onlookers.