Mosul offensive: ISIS militants fleeing to Syria, says tribal leader

* Shelling resumes in the town of Bashiqa on the outskirts of Mosul

* ISIS executed about 40 people celebrating the "liberation" of their villages by Iraqi forces, a Mosul official said

Coalition forces celebrated better-than-expected territorial gains over the weekend and artillery fire pummeled ISIS positions in the encircled town of Bashiqa early Monday morning in the relentless push for Mosul.

According to the Iraqi Joint Operations command center, 78 towns and villages have been liberated so far as the operation to retake the city enters its second week.

The center said 772 ISIS fighters had been killed and 23 were detained, 127 vehicle-borne explosive devices were destroyed, two bomb-making factories were discovered and nearly 400 improvised-explosive devices were remotely detonated so far.

The offensive is remarkable for both its speed and the level of cooperation that this disparate group is showing in the face of its common enemy -- an extraordinary union of factions that have long stood on opposing sides in Iraq's history, with Kurdish forces, Christians and Shia Muslims fighting alongside the majority Sunni Arabs.

The thousands of ground troops were supported from above with a concentrated program of airstrikes aimed at weakening ISIS' defenses -- the highest weekly number since the campaign against the terror group began, according to Brett McGurk, Special Presidential Envoy for the Global Coalition to Counter ISIL.

"One week into #Mosul operation, all objectives met thus far, and more coalition airstrikes than any other 7-day period of war against #ISIL," he wrote on Twitter, using another name for ISIS.

The coalition force, which vastly exceeds ISIS' numbers, is closing in on the beleaguered city, still home to an estimated 200,000 to 300,000 civilians.

But the coalition is well aware that resistance -- already tough in the open fields and small villages surrounding the main prize -- is likely to ramp up significantly when the city's perimeter is breached.

ISIS has been in control of Mosul for two years, giving its fighters plenty of time to fortify defenses, and the militants have time and time again proved themselves adept at bloody, urban warfare.

The city was important to the terror group as the cultural capital of its envisaged caliphate, or Islamic state.

With this weekend's gains have come pockets of horrific losses.

ISIS executed about 40 people who were celebrating the apparent liberation of their villages by Iraqi forces, a Mosul City Council official said Sunday, citing local sources.

The official said that although Iraqi troops passed through the village where the executions took place -- near Nimrud, south of Mosul -- they did not leave units behind to ensure that ISIS militants stayed out.

These follow executions on Thursday and Friday, when ISIS militants rounded up and shot dead 284 men and boys, an Iraqi intelligence source told CNN.

Emergency crews have been working around the clock to extinguish a fire at a sulfur factory in Qayyara, about 30 kilometers south of Mosul, that was torched by ISIS militants.

The fire, started when the ISIS militants left explosives and slow-burning oil in sulfur deposits and around the facility, has sent plumes of toxic smoke in the air, causing hundreds to seek medical help.