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 air strike took place on Thursday morning near the town of al-Bab, which Turkish troops and Turkey-backed Syrian opposition fighters are trying to capture from Isis. News of the deaths came as Turkey was holding funerals for five Turkish soldiers killed in an Isis attack the day before.

The Turkish military said Turkey and Russia were conducting a joint investigation into the incident.

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

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

The attack in the Hamadaniya neighbourhood, also wounded seven other volunteers, the organisation said. The Britain-based Syrian Observatory for Human Rights said there were also other mortar rounds that hit security checkpoints in the area on Wednesday. Syria's state news agency said mortar shells also hit other buildings in the neighbourhood.

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.

The cease-fire, in place since 30 December, has excluded areas where militant factions, Isis 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 Isis-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, one if Isis's last remaining strongholds in northern Syria.

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

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.