Donetsk, Ukraine (CNN) -- Intense fighting broke out in eastern Ukraine on Monday when government air and ground forces attacked pro-Russian separatist gunmen who had seized an airport terminal.
The clashes between Ukraine's Army and separatists in Donetsk have caused some casualties, according to a statement posted on the official website of the city's mayor, which also advised residents to stay in their homes.
A CNN team in the city observed a Ukrainian combat helicopter flying over the Donetsk airport and firing at something. It was unclear what the target was. Black smoke billowed up from the area.
Pro-Russian separatists shot at the helicopter as it flew overhead.
Gunfire and explosions could be heard in the city.
The fighting marked the worst violence that this key population center in eastern Ukraine has seen since the start of the crisis. And the violence came hours after newly elected Ukrainian President Petro Poroshenko said he'd potentially like to negotiate a way out of the crisis.
Authorities canceled all flights after separatists stormed and seized Donetsk's airport terminal, according to the national news agency of Ukraine, Ukrinform. The separatists ignored a government ultimatum to vacate the premises, Ukrinform reported.
After a Ukrainian military plane "made a preliminary shot," paratroopers landed and began clearing the airport, Ukrinform reported. In the fighting, a separatist anti-aircraft gun was destroyed, Ukrinform said.
Though the Ukrainian government said it was taking control, CNN reporters who witnessed the action saw separatist reinforcements moving toward the airport in trucks.
Both sides later claimed they held the airport.
Government officials claimed flights would resume by 9 a.m. on Tuesday.
The State Border Service of Ukraine said that up to 40 trucks with gunmen had been spotted on Russian territory within 10 kilometers of Donetsk.
Nick Paton Walsh and Andy Care reported from Donetsk. Ralph Ellis wrote and reported from Atlanta. Victoria Butenko and Elena Sandryev contributed to this report.