ANKARA, Turkey (AP) — A Russian warplane "accidentally" hit a building on Thursday in northern Syria with Turkish soldiers inside, killing at least three troops and wounding 11, Turkey's military said.

President Vladimir Putin promptly called his Turkish counterpart, Recep Tayyip Erdogan, to express regrets over the friendly fire incident. The Kremlin said Putin conveyed his condolences over the "tragic incident."

The Russian airstrike took place in the morning, near the town of al-Bab, which Turkish troops and Turkey-backed Syrian opposition fighters are trying to capture from the Islamic State group. News of the deaths came as Turkey was holding funerals for five Turkish soldiers killed in an IS attack near al-Bab the day before. The Turkish military said Turkey and Russia were conducting a joint investigation into the incident.

Meanwhile, in a reflection of how complex the Syrian theater of war has become, Turkey-backed Syrian opposition fighters and Syrian government allied troops briefly clashed on the outskirts of an Islamic State-held town in northern Syria.

It was the first such confrontation since the Turkey-backed offensive began in northern Syria last August. Thursday's clashes came despite Ankara's remarks the previous day that it was in talks with Moscow to coordinate troop movements and avoid any encounter with the Syrian military in the fighting around the IS-held town.

Turkey and Russia recently repaired ties that were strained by Turkey's downing of Russian jet near the border with Syria two years ago. In late December, the two countries brokered a cease-fire for Syria and in January they sponsored peace talks in Astana, Kazakhstan. Syrian rebels and President Bashar Assad's government officials attended that gathering.

Thursday's airstrike deaths raise the number of Turkish troops killed in Turkey's operation in Syria to 64.

Syria's opposition-run Thiqa news agency, which covers Syrian opposition fighters in the offensive on al-Bab, told The Associated Press that a sniper opened fire on the Turkey-backed forces advancing on the western edge of al-Bab, sparking clashes that wounded several fighters.

The Britain-based Syrian Observatory for Human Rights said the clashes were fierce around the southwestern edge of al-Bab, where the government troops made a significant advance on Thursday.

The Turkey-backed opposition fighters have also made a significant push into al-Bab, seizing a strategic hill on the town's western edge. There was no immediate comment from Damascus on the reported clashes between the government troops and the opposition fighters.

A Russia- and Turkey-sponsored cease-fire, in place since Dec. 30, has excluded areas where militant factions, the Islamic State group and Syria's al-Qaida affiliate, operate or hold ground.

Syrian government troops and allied militias, as well Turkish troops, the U.S-led international coalition, and Russia have been going after IS in different parts of the country.

The push has lately focused on the IS-held town of al-Bab, northeast of Aleppo. Syrian government forces and their allies have been pushing from the south, aided by Russian airstrikes

On Thursday, government troops seized a village south of al-Bab, bringing them less than 3 kilometers (1.5 miles) from the town, according to opposition monitors.

Meanwhile, Turkish troops, backing Syrian opposition fighters, have pushed their way from the western part of the town, entering the outskirts of al-Bab. The two advances have effectively encircled the militant group, which had set up strong fortifications around al-Bab.

Another fight against IS is raging in east and northeast Syria, where Kurdish troops, backed by the United States, are advancing against the IS in the militants' de facto capital of Raqqa. Meanwhile, government forces are battling an IS offensive in Deir al-Zour and another one in the central Homs province, around the ancient town of Palmyra.

In other developments in Syria, the Syrian Arab Red Crescent and the Red Cross said a suspected rebel mortar attack that hit a Red Crescent distribution center in the government-held city of Aleppo Wednesday killed a volunteer and two civilians.

The attack in the Hamadaniya neighborhood also wounded seven other volunteers. The Observatory said there were also other mortar rounds that hit security checkpoints in the area. Syria's state news agency said mortar shells also hit other buildings in the neighborhood.

After years of heavy fighting, Syrian government forces drove the rebels out of eastern Aleppo in December, but the opposition still holds some areas on the city's outskirts. Fighting has continued around Aleppo and in other parts of Syria despite a Russia- and Turkey-sponsored cease-fire.

Violence has also beset rebel- and opposition-held areas, including in Homs, where at least nine civilians were killed in suspected Russian or government airstrikes on Wednesday. The governor of Homs, Talal Barazi, told the AP that three people were killed when an explosive device was detonated in the city on Thursday.

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El Deeb reported from Beirut. Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.