BEIRUT (AP) -- The U.S. military says a misdirected airstrike this week killed 18 allied fighters battling the Islamic State group in northern Syria.
U.S. Central Command said Thursday that coalition aircraft were given the wrong coordinates by their partner forces, the predominantly-Kurdish Syrian Democratic Forces, for a strike intended to target Islamic State militants south of their Tabqa stronghold.
The strike hit an SDF position instead, killing 18 fighters. Central Command says the incident occurred on Tuesday.
The SDF, with the help of air and ground support from the U.S.-led coalition, has surrounded Tabqa.
Several nations have lent their air power to the coalition to defeat the Islamic State group. It is not clear which air force was behind the strike.
Meanwhile, Russia's foreign minister says Moscow expects the United Nations' chemical weapons watchdog to conduct an extensive probe into last week's chemical attack in Syria.
Sergey Lavrov said Thursday that inspectors from the Organization for the Prohibition of Chemical Weapons should both visit the Syrian air base, which the U.S. said had served as a platform for the attack, and the town of Khan Sheikhoun that was hit to get a full and objective picture.
He said Russia vetoed a Western draft U.N. resolution Wednesday because it failed to mention the need to inspect the area of the attack.
The U.S. blamed the Syrian government for launching the attack, but Russia claimed that the victims were killed by toxic agents released from a rebel chemical arsenal hit by Syrian warplanes.
A British delegation at a behind-closed-doors meeting of the international chemical weapons watchdog says in a tweet that the organization's director general has said that its investigators already are testing samples from a suspected deadly chemical weapons attack in Syria's Idlib province.
The British delegation tweet says that Organization for the Prohibition of Chemical Weapons Director-General Ahmet Uzumcu told Thursday's hastily convened meeting of its executive council that the "Fact Finding Mission is working to gather evidence" about the April 4 attack in Khan Sheikhoun that killed nearly 90 people.
It says that samples already are being tested and the mission is expected to report its findings in three weeks.
The OPCW's Fact Finding Mission in Syria investigates alleged attacks, but does not apportion blame.
U.S.-backed Syrian Kurdish forces say they have entered the fourth stage of their campaign to capture the Islamic State group's de facto capital Raqqa with an advance on the militants in a valley north of the city.
The Syrian Democratic Forces, which include Arab fighters, say they are working to clear the militants out of the Jalab Valley, north of Raqqa. An estimated 300,000 people are in the city.
The SDF says it wants to isolate Raqqa before attacking it. Their closest position is within 8 kilometers (5 miles) northeast of the city. But the countryside south of Raqqa is still under IS control.
It is unclear how many stages are planned for the SDF campaign.