While the United States supports Turkey's concern about a safe and secure Turkish-Syrian border, military operations by Turkey into northeast Syria will not advance regional stability, the State Department said Friday.
Turkey was reportedly intensifying the shelling into Syria's Kurdish-controlled Afrin region.
"We do not believe that a military operation, whether in Afrin or directly against the self-defense Syrian Democratic Forces (SDF), in the north or northeast of Syria, serves the cause of regional stability, Syrian stability or indeed Turkish concerns about the security of their border," a State Department official said, adding that he could not comment further without more information about Ankara's reported operations.
Turkey's threat to intervene in Afrin came after a U.S.-led coalition said it would form a 30,000-strong Kurdish-led border security force in northern Syria.
Washington later said the effort had been mischaracterized and that the U.S. was not creating a border force, but that the coalition would provide security to liberated areas, blocking escape routes for Islamic State militants.
The United States has led a coalition carrying out airstrikes against IS targets in Syria and Iraq since 2014, and the Pentagon said last month that there were about 2,000 U.S. military personnel in Syria.
Despite suffering apparent defeats on the ground in Iraq and parts of Syria, IS is far from dead.
"ISIS is still present" and a lethal force, the State Department official said using an acronym for the group. "The military campaign against the so-called caliphate is not over. There is heavy fighting. ... ISIS in northern Syria and Iraq have chosen not to fight and die but move out of the combat area."
Some experts warned that even after areas in Syria have been liberated from IS, the country will need a broader political settlement that reflects regional and national realities to bring displaced people home.
"A Syrian political settlement and the refugee crisis should not be addressed separately," said Kheder Khaddour, a scholar at the Carnegie Middle East Center in Beirut.
Khaddour added that "a settlement without a refugee return will hinder reconstruction by keeping away needed professionals and civil society actors. A return without a settlement will lead to local conflicts between traditional leadership and emerging ones empowered during the war."
The State Department is facilitating Syrian refugees' safe return home by de-mining and restoration efforts.
Earlier this week, Secretary of State Rex Tillerson laid out Washington's objectives for a "whole and unified sovereign Syria." He announced Wednesday a revitalized diplomatic and military strategy in Syria, including the defeat of IS and al-Qaida; a U.N.-led political process under a post-Bashar al-Assad Syria that is stable, unified and independent; diminished Iranian influence; conditions that allow refugees to return; and a country free of weapons of mass destruction.