DAMASCUS, Jan 21 (Reuters) - Syria's government said a drone strike by the Kurdish-led Syrian Democratic Forces killed seven of its soldiers on Wednesday, though the SDF denied this, in an incident that threatens to derail a ceasefire after days of fighting in the northeast.
The Syrian army called the attack, which it said took place as soldiers were securing a captured military base containing explosives, a dangerous escalation.
The SDF, previously the main U.S. ally in Syria, said it had not carried
out a strike and that the blast had occurred when Syrian soldiers were moving explosives. It accused the Syrian army of violating the truce with attacks in several locations.
After days of rapid gains, the government on Tuesday said it had reached an understanding with the SDF for it to agree a plan over a four-day ceasefire to integrate into the central state, and that otherwise the SDF would face an assault on the two last main cities it holds.
FOUR-DAY CEASEFIRE
The government advance against the SDF has put into doubt years of Kurdish de facto autonomy in the northeast and is significant for Syria's ties with the United States and Turkey, and for the fate of thousands of detained Islamist militants.
Turkish President Tayyip Erdogan, a main ally of Syrian President Ahmed al-Sharaa, said on Wednesday that the SDF, which it regards as a terrorist group, must lay down arms and disband now to avoid further bloodshed.
The U.S., which backed the SDF as its main ally in driving Islamic State militants from swathes of Syria in bloody fighting a decade ago, on Tuesday urged the group to accept the government's offer.
The U.S. said the reasons for its partnership with the SDF had expired, but that it was still concerned about the fate of thousands of detained Islamic State militants and civilians associated with them in facilities guarded by the SDF.
The SDF said on Tuesday it accepted the ceasefire and that it would not engage in any military action unless attacked. SDF leader Mazloum Abdi had earlier said it regarded the protection of Kurdish majority areas as a "red line".
Wedged into a triangle between the Turkish and Iraqi borders, northeast Syria has areas with both Arab and Kurdish majorities and contains most of Syria's energy reserves.
Syrian troops were still positioned outside the last main Kurdish-held cities in the northeast, Hasakah and Qamishli, on Wednesday, Reuters reporters in the area said.
They had brought significant reinforcements the previous evening, with convoys of tanks and other military vehicles, as well as buses packed with fighters, arriving late into the night.
The troops had frozen their advance after Sharaa's new ceasefire announcement the previous evening, and were waiting for further orders that would be shaped by the SDF's reply to Sharaa's proposal.
RELATIONS WITH TURKEY, U.S.
The strategic picture in Syria has changed entirely over the past 13 months since rebels under former al Qaeda commander Sharaa ousted Iran-backed president Bashar al-Assad in a lightning advance that stunned the region.
The SDF, which had at times fought Sharaa's rebels during Syria's civil war, had controlled around a quarter of Syria where it maintained autonomous rule from Assad in Damascus.
The U.S. maintained a small military presence in the area and helped the SDF fend off attacks by Assad and allied forces. However, Sharaa has developed good ties with Washington, changing its relationship with the Damascus authorities.
Turkey, which regards the SDF as affiliated with the Kurdistan Workers Party (PKK) that has waged a decades-long insurgency inside Turkey, had a military presence in other parts of northern Syria in support of rebel groups there.
Ankara and the PKK are now engaged in a peace process and Turkey regards the end of SDF control in Syria as a vital part of that.
Speaking to parliament on Wednesday, Erdogan welcomed the declared ceasefire in Syria, saying he hoped the group's "full integration" into the Syrian state would herald a new era in Syria.
Erdogan and Trump discussed Syria in a phone call overnight, touching on the situation of prisoners in Syrian jails and the continued fight against Islamic State.
(Reporting by Ahmed Elimam and Reuters in Syria; Writing by Angus McDowall; Editing by Aidan Lewis)
