ISTANBUL — Three Turkish soldiers were killed and four were wounded in a missile attack Tuesday by the Islamic State group in northern Syria— the first Turkish casualties caused by the militants in Turkey’s two-week-old incursion into Syria.

The Turkish fatalities came after Turkish troops and allied Syrian rebels on Sunday expelled IS from the last strip of territory the militant group controlled along the Syrian-Turkish border, effectively sealing the extremists’ self-styled caliphate off from the outside world.

Turkey launched the incursion into Syria — the so-called Euphrates Shield operation — to back Syrian rebels in their fight to push IS out of the town of Jarablus and to limit the Syrian Kurdish forces’ advance west of the Euphrates River.

In a statement, Turkey’s military said the militants fired rockets at Turkish tanks during clashes near the border area from where IS was pushed out of on Sunday, immediately killing two and wounding five soldiers. It said the wounded were evacuated by helicopters. One of the wounded soldiers died despite efforts to save him, according to Turkey’s state-run Anadolu news agency.

The military said two Turkey-backed Syrian rebels were killed and two wounded rebels were also evacuated.

The Britain-based Syrian Observatory for Human Rights said there were intense clashes on Tuesday between the Turkish-backed rebels and IS militants east of the town of al-Rai and surrounding villages.

The territorial losses at the border were the biggest blow to the militant group, which also has suffered a series of recent battlefield setbacks elsewhere in Syria and in neighboring Iraq.

The three killed by IS were not Turkey’s first casualties following the launch of the incursion, though they were the first fatalities at the hands of the militant group since the operation began.

On the fifth day of the operation, a Turkish soldier was killed in clashes with Kurdish fighters in northern Syria.

In July 2015, a Turkish soldier was killed after IS militants shot across the border into Turkey. Ankara conducted airstrikes against IS inside Syria after that.