(CNN) - Defense Secretary Jim Mattis has called on Turkey to show restraint in its military incursion in northern Syria, warning that violence on the border could create a vacuum for ISIS and Al Qaeda militants, Reuters reports.
Turkey launched an operation targeting Kurdish fighters in Afrin on Saturday, opening new front lines in the seven-year Syrian civil war, just as ISIS fighters in the country have been all but defeated.
The Turkish military said in a statement Tuesday it had killed 260 Kurdish militia and ISIS fighters in Afrin. That claim cannot be independently verified and CNN has no indication there are ISIS fighters in the area Turkey is targeting.
Speaking during an official visit to the Indonesia capital, Jakarta, Mattis said the situation in Afrin "could be exploited by ISIS and Al Qaeda" now that the focus was off them.
"And obviously it risks exacerbating the humanitarian crisis that most of Syria is going through," he said.
"In the Afrin area, we had actually gotten to the point where humanitarian aid was flowing, refugees were coming back in ... the Turkish incursion disrupts that effort."
But Mattis also said that Turkey had legitimate security concerns, Reuters reported, echoing remarks made by US Secretary of State Rex Tillerson on Monday.
Ankara, which has long fought Kurdish unrest in southeastern Turkey, is determined to prevent the establishment of a Kurdish state at Turkey's border with Syria. It has used military force for decades in the country's southeast, and has also carried out strikes over the Syrian border.
The Kurdish People's Protection Units (YPG) control a large part of northern Syria -- some 24% of all Syrian territory, including hundreds of kilometers along the Turkish border.
US President Donald Trump is due to speak to his Turkish counterpart, Recep Tayyip Erdogan, on Wednesday.
White House spokeswoman Sarah Sanders said Trump will call on Turkey to "de-escalate" the situation, reiterating comments she gave on Monday about remaining "focused on the goal of defeating ISIS, de-escalating and resolving the Syrian conflict, and protecting innocent civilians."
The complex Syrian conflict has pitted Turkey and the US, two NATO allies, against each other through proxy.
Turkey is targeting the YPG, which Ankara says is a terrorist organization and a mere extension of the Kurdistan Workers' Party (PKK). Turkey, the US and EU all consider the PKK a terrorist organization.
But Washington backs and arms the YPG in other parts of the country as a leading force in the fight against ISIS.
The links between the US and YPG in Afrin are looser than in other parts of the country, but the group poses problems for Turkey-US relations nonetheless.
CNN reporters in the Turkish town of Reyhanli near the Syrian border said they heard regular artillery shelling continuing Tuesday, and understood that a front line around Mount Barsaya was shifting back and forth between the Turkish-backed Free Syrian Army rebel group and the YPG.
The Turkish military said Tuesday that two of its soldiers had been killed so far in the operation. The exact number of people killed and injured is not clear, but there are believed to be casualties on both sides.
Tillerson denied on Monday that clashes between the militia groups would hurt Turkey-US relations.
"I don't think you're going to find two NATO allies facing off at all," Tillerson told reporters.
But Erdogan struck a less diplomatic tone, challenging the US on its support for the YPG.
"Why are you fighting against ISIS with another terrorist organization? Why not carry it out with your strategic partner?" he said.
In the four days of the new operation, Turkish authorities have detained more than 90 people across the country for promoting "terrorist propaganda," state media Anadolu reports. Many were accused of doing so through social media, in posts related to the incursion.
The arrests come in a wider purge in the country following a failed military coup in 2016. Erdogan's government has carried out mass detentions, gutting public institutions and quieting the political opposition.
Among those who have been detained in Turkey's crackdown is a regional pro-Kurdish politician, as well as people accused of spreading propaganda on public transport.