US Defense Secretary Jim Mattis urged Turkey on Tuesday to exercise restraint in its military operations in northern Syria, which he said had disrupted the peaceful return of refugees and could prove to be an opening for al Qaeda and Islamic State."This could be exploited by ISIS and al Qaeda, obviously, that we're not staying focused on them right now. And obviously it risks exacerbating the humanitarian crisis that most of Syria is going through," Mattis told reporters during a trip to Indonesia, using an acronym for Islamic State.Turkey's four-day-old campaign aims to crush US-backed Kurdish YPG fighters in an air and ground offensive on Syria's Afrin region and has opened a new front in Syria's multi-sided civil war.Mattis said Afrin had been stabilizing, prior to the Turkish military operation."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," Mattis said.Ankara has long been infuriated by US support for the YPG, which it sees as a domestic security threat, one of several issues that have brought relations between the United States and its Muslim NATO ally close to the breaking point.The United States hopes to leverage the YPG's control and that of US-backed Arab fighters in northern Syria to give it more diplomatic muscle as it tries to revive UN-led talks in Geneva on a deal that would end Syria's civil war.A key US negotiating position is that there must be a political transition away from Syrian President Bashar al-Assad, whom Washington blames for the brutal conflict and accuses of using chemical weapons on his people.Mattis said Turkey had legitimate security concerns, even as he reaffirmed the US strategy in Syria."We’ve had our disagreements (with Turkey). But at the same time, I would just say that it is much better for Turkey and for the Kurds and for the Sunnis that we have the Americans in a position to influence the situation rather than Assad," Mattis said.