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 .
Clashes have caused some casualties , a statement from the mayor 's office says . Authorities advise residents to stay in their homes as violence erupts . - The government has declared three days of national mourning for those killed and injured during protests on Friday. In response , Ukrainian air and ground forces attacked the airport .