The Observatory, which gathers its information from an activist network on the ground, said the fighter jets hit a car carrying the foreign militants in a location near a village called Faouj al-Maylabeih in the southern countryside of the city.
The monitor said it was not clear whether the air strike in Hasaka province was carried out by the Syrian army or a U.S.-led coalition that is mounting a separate aerial campaign against Islamic State in Syria.
The United States said earlier this month it had intensified an aerial campaign against Islamic State in Syria with a wave of strikes in and around Raqqa city, the de facto capital of the militants’ self-declared caliphate in parts of Syria and Iraq.
The Syrian army and militia fighting alongside it are mounting a separate campaign against Islamic State in the region that borders Turkey to the north and Iraq to the east.
The ultra-hardline Islamic State is fighting in Hasaka province against both pro-government forces and Syrian Kurdish fighters who have received air support from the U.S.-led coalition.
They have also continued to stage lightning attacks inside the city of Hasaka, although they were driven out of some districts after they mounted a major offensive that failed last month.
That offensive attempted to capture the provincial capital of the oil and grain producing province of Syria.
The city is divided into zones run separately by the government of President Bashar al Assad and a Kurdish administration.