Satellite image of Syria, 2015.

DAMASCUS — Turkey has closed two border crossings with Syria as a security precaution as fighting around the northern Syrian city of Aleppo intensifies, Turkish customs and government officials said on Wednesday.

The crossings at Oncupinar and Cilvegozu in Turkey’s southern Hatay province have been shut to vehicles and individuals crossing from Syria since Monday, customs officials at both posts told Reuters.

“Turkey has some security concerns and it is natural for measures to be taken based on the threat assessment conducted. This is what is also expected by Turkey by the international community,” said an official at a government agency, who declined to be identified.

He did not say when the crossing would be reopened.

Humanitarian aid will not be affected, the government official said. Syrians with passports are still allowed to cross into Syria.

Turkey has kept its borders open to refugees since the start of Syria’s civil war four years ago, but it has come under criticism for doing too little to keep foreign fighters crossing and joining militant groups including “Islamic State”.

Thousands of foreigners from more than 80 nations including Britain, China and the United States have joined the ranks of Islamic State and other Islamist groups in Syria and Iraq, many crossing through Turkey.

On Wednesday, Ankara said it had detained 16 Indonesians from three families who were trying to cross into Syria.

Aleppo, around 30 miles south of the border, is divided between government forces and insurgent groups fighting to topple President Bashar al-Assad in a conflict estimated to have killed 200,000 people.

Dozens of people were killed last Wednesday when insurgents attacked a Syrian government security building in the city, bombing it and then launching a ground assault.

The closure of the Turkish border posts also comes after an air strike on Sunday in northwestern Syria, close to the border, hit a camp used by al-Qaeda-linked Nusra Front.

The main Western-backed political opposition said on Thursday that U.S.-trained rebels due to fight Islamic State will fail if they are not also protected from the Syrian army, after Washington said it might lack the legal authority to defend them from Syrian troops.

A fledgling U.S. military-led training program starting in coming weeks aims to train upwards of 5,000 fighters a year to battle the hardline militant group.

But U.S. Defense Secretary Ash Carter said on Wednesday that the United States did not appear to have clear-cut legal authority to protect Syrian rebels against attacks by Syrian government forces.

Monzer Akbik, a representative for the National Coalition, based outside Syria, said mainstream rebels had repeatedly found themselves attacked by Syrian forces while confronting Islamic State, and that the same would happen to U.S.-trained rebels.

“(The rebels) don’t have anywhere to go, they have to fight on two fronts … if they are attacked from the rear by the regime, it will undermine their fight against ISIS, leading to a long war of attrition rather than a war that you can win.”

Akbik said the Western-backed Free Syrian Army rebel alliance had pushed IS back from parts of northwest Syria in early 2014, but had then been pummeled simultaneously by Islamic State and Syrian forces after the Islamists regrouped with supplies from neighboring Iraq.

Washington says Assad has lost legitimacy, but it has in effect made common cause with him by leading air attacks against the IS insurgency with Damascus’s tacit blessing, forcing it to tread very warily.

Assad said in January that “any troops that don’t work in cooperation with the Syrian army are illegal and should be fought.”

It is not clear what relationship the new force, which will be trained in Jordan, Turkey, Saudi Arabia and later Qatar, will have to the U.S.-led regional coalition, which has provided air support to anti-Assad rebels fighting alongside Kurdish forces in Syria against IS.

Akbik said the coalition should also defend the new force against Assad’s forces.

Darkness

Satellite images highlight the devastation caused by four years of conflict in Syria, with most of the country now plunged into darkness.

Eighty-three percent of the lights have gone out in Syria as a humanitarian crisis continues to grip the country, according to scientists at the Wuhan University in China.

In Aleppo, one of the cities hardest hit by civil war and Isis’s insurgency, a staggering 93 percent of the city is now in the dark.

Damascus has been under government control but is now 35 percent darker than before the war. But the capital’s suburbs, where fighting has been fierce, are 63 percent darker.

In Deir al-Zour and Raqqa, which are largely controlled by IS, the group has struggled to keep power flowing, especially after the coalition airstrike campaign that began in August last year.

Around 14 million children are suffering hardship and trauma from the war in Syria and Iraq, the United Nations children’s agency said on Thursday, highlighting the needs of children struggling to cope with severe violence, and the danger to the rest of the world of failing to help a generation preyed on by extremist groups.

“Violence and suffering have not only scarred their past, they are shaping their futures,” Anthony Lake, Unicef’s director, said in a statement released on Thursday with a report on the plight of 5.6 million children in Syria and two million more who have fled as refugees. Close to three million children in Iraq and 3.6 million children in neighboring countries bearing the brunt of the influx are affected by the conflict, Unicef estimated.