DONETSK -- If the fighter jets roaring overhead weren't enough of an indication that the situation in Donetsk had changed dramatically, the thundering air strikes that followed erased any doubt.
It was the Ukrainian military's biggest and most rapid response since the unrest began here in the east three months ago.
Their target: pro-Russian separatists who had launched an audacious attempt to seize the city's international airport.
The separatists made their first move on the airport in the dead of night. Pictures emerged of heavily-armed militants in the airport terminals and holding positions on the roof.
By mid-morning, rebel reinforcements were arriving by the truckload, spreading out among the buildings and trees which line the airport perimeter.
As a group of separatist fighters took cover in a cluster of trees at the end of an on-ramp to the airport, three helicopter gunships seemingly came out of nowhere and opened fire, tearing into the woods and the grassy area in front.
The fighters on the ground tried to fight back, spraying gunfire in every direction.
The Ukrainian military operation -- using jet fighter air strikes, helicopter gunships, heavy artillery and a ground assault by Ukrainian paratroopers -- was unprecedented.
Bodies piled up at the morgue. Rebel commanders have said they lost more than 100 fighters. The Ukrainian government said its forces suffered no losses in the assault.
The battle for the airport may prove pivotal for two reasons: it was the highest-profile attack separatists have launched so far, and it triggered the highest-profile response from the Ukrainian military so far.
Pro-Russian separatists have had the run of this region for a number of weeks. Armed gunmen roam the streets freely. There has been virtually no Ukrainian troop presence in Donetsk, and while there are still Ukrainian police here, neither side pays the other much attention -- the local police represent no challenge to the heavily armed gunmen.
Separatists still control important government buildings, and they made good on their vow to ensure that not a single vote was cast here in the country's presidential elections.
While president-elect Petro Poroshenko has called for peaceful negotiations with the separatists, he's compared the armed pro-Russians to "Somali pirates," and said the crisis should be resolved in a matter of hours, not months.
Now Donetsk is bracing for what comes next. Shops and restaurants are closed. Rumors are rife that Ukrainian military units are getting ready to launch an invasion to retake the city.
In an emergency televised address on Tuesday, the Donetsk's mayor warned people to stay indoors, while assuring nervous residents that government forces would not push into the center of the city. At night, this normally bustling city of more than 1 million people, seems virtually deserted.
The large-scale military response signaled a significant change in tactics.
Ukraine's government has been reluctant for months to unleash its heavy weapons against pro-Russian separatists, for fear of provoking a response from Moscow. There are still tens of thousands of Russian troops massed near the Ukrainian border, but U.S. officials told CBS News on Tuesday that about 6,000 of the approximately 40,000 troops near the border had returned to their home bases, and a couple thousand more are getting ready to move.
Moscow announced the planned withdrawal of troops from the border area several days ago, but officials said it could take almost a month for the repositioning to be completed.
The separatists won't last long without Russian backing. On Tuesday, the separatist leader in Donetsk, Denis Pushilin, made a direct appeal to Putin for help. He may be feeling more isolated, and increasingly desperate.
The Russian government has demanded an immediate halt to Ukraine's military offensive in the east, but the repositioning of Russian troops away from the Ukrainian border sends a message.
And Ukrainian deputy prime minister Vitaly Yarema said Tuesday that the "anti-terrorist operation" would continue in the east, "until all the militants are annihilated."