Russian President Vladimir Putin has apologized to Turkish President Recep Tayyip Erdogan for a Russian airstrike that both governments claim accidentally struck a building in Syria occupied by Turkish soldiers.

Most reports, and the official statement from Turkey’s government, say three Turkish troops were killed and another 10 or 11 wounded in the incident, although EuroNews states there were five fatalities.

The Turkish military has claimed “significant progress” in liberating the key town of al-Bab from the Islamic State. “Currently the town is besieged from every side. The outer districts of the town have been brought under control,” Turkish Prime Minister Binali Yildirim announced on Wednesday.

The troops killed by Russia’s airstrike were supporting Syrian rebels in the battle against ISIS around al-Bab, with the ultimate goal of driving both the Islamic State and Kurdish forces farther away from the southern border of Turkey, as the BBC reports.

The Russians say they were attempting to strike ISIS positions when they hit the building occupied by Turkish forces. The Kremlin stated that President Putin “expressed condolences over a tragic incident which resulted in the deaths of several Turkish troops in the al-Bab area” in a phone call to President Erdogan.

Turkey’s Daily Sabah quoted a Turkish military statement that the airstrike occurred around 8:40 A.M. on Thursday. Both Turkey and Russia have announced they will investigate the incident thoroughly.

Turkey’s relations with Russia have improved greatly since the downing of a Russian jet on the Turkish border last year. There may still be some disagreements over long-term strategy in Syria, as the Russians have acted to support the regime of President Bashar Assad, while Turkey has long sought his removal from power. In fact, when Turkish forces moved into the al-Bab area, Erdogan explicitly stated they were there to “end the rule of the cruel Assad, who has been spreading state terror.”

However, Russian media has lately been talking up a “U-turn” by the Turks and quoting Assad’s assurances that “the Turkish president’s position is positive for Syria.” The relationship between Ankara and Damascus will be clarified soon since Syrian forces are also advancing on the city of al-Bab.

Turkey is also seeking improved relations with the United States. As EuroNews reports, President Erdogan has been touting a cooperative plan for capturing the ISIS capital of Raqqa in Syria, as well as plans for the sort of Syrian “safe zones” envisioned by U.S. President Donald Trump.

An Erdogan spokesman said Turkey’s mission in Syria “has been helped by increased coordination with the US-led coalition which, in the last 10 days, has conducted air strikes on ISIL targets.”