Turkey and its allied Syrian National Army continued to amass forces in northern Syria on Thursday ahead of the imminent launch of a Turkish offensive against the Kurdish Syrian Democratic Forces in Tal Rifaat and Manbij west of the Euphrates River.
Turkey continued to send reinforcements to the frontlines in Manbij, Tal Rifaat and Ain al-Arab, also known as Kobane.
The Syrian opposition, and for the third straight day, carried out drills using live ammunition.
A source from the Syrian National Army said the factions in northern Aleppo and the Raqqa countryside have nearly completed their military preparations and are waiting zero hour.
Turkish President Recep Tayyip Erdogan declared on Thursday that his country will continue to pursue "terrorist hideouts", a reference to the Kurdish People's Protection Units (YPG), the military backbone of the SDF.
Russia, expressing alarm, said it hoped Turkey "refrains from actions which could lead to a dangerous deterioration of the already difficult situation in Syria".
A foreign ministry spokeswoman told Interfax news agency that if Ankara launched an attack it would represent a direct violation of the sovereignty and territorial integrity of Syria and provoke further escalation of tension.
SDF commander Mazloum Abdi called on all sides to "prevent any new tragedies and support de-escalation", warning that a new assault would fuel yet more displacement in Syria's 11-year-long conflict.
A new offensive would create a humanitarian crisis and undermine the SDF campaign against ISIS, he added.
Turkey has mounted four operations in northern Syria since 2016.
While Turkey views the Kurdish-led forces in Syria as terrorists and a national security threat, the United States views the SDF as an ally that has helped drive ISIS from vast areas of Syria.
Washington, whose support for the SDF has long been a point of tension in ties with its NATO ally Turkey, has expressed concern, saying any new offensive would put at risk US troops - which have a presence in Syria - and undermine regional stability.
On a visit to the Turkish town of Hatay near the Syrian border, the US ambassador to the United Nations reiterated US opposition to any military action.
"We think that nothing should be done to break the ceasefire lines that have already been established," Linda Thomas Greenfield said.
She added any such action would not only increase suffering but also the number of displaced people, including some who might try to cross into Turkey.
Russian army helicopters have been making unusually frequent flights over northern areas held by the government or SDF over the last week, the Syrian Observatory for Human Rights, a UK-based organization which reports on the conflict, said.