BEIRUT, Lebanon — A coalition of Syrian insurgent groups said it had begun a major offensive on Friday to break the monthslong siege of eastern parts of Aleppo by government and Russian forces, and at least 15 people were reportedly killed in fighting in government-controlled parts of the city.

Observers reported heavy gunfire, mortar fire, car bombs and other explosions. Black smoke from burning tires rose above the city, evidently in an attempt to obscure the view of pilots flying Russian warplanes overhead.

The new offensive was a strong sign that rebel groups vetted by the United States were continuing their tactical alliances with groups linked to Al Qaeda, rather than distancing themselves as Russia has demanded and the Americans have urged.

Russia has reserved the right to attack anyone working with the Qaeda-linked groups, saying there is little distinction between them. The rebels argue that they cannot afford to shun any potential allies while they are under fire, including well-armed and motivated jihadists, without more robust aid from their international backers.