Separatists Near Donetsk Airport

UPDATED: 12:05 p.m. ET

DONETSK, Ukraine — Ukraine government forces launched an aggressive assault Monday on pro-Russian separatists who had earlier seized a terminal building in the Donetsk airport.

The government forces used airstrikes and helicopters to battle the insurgents, not long after President-elect Petro Peroshenko vowed to take a tougher approach to the armed militants who have seized government buildings in a succession of cities in the Russian-speaking eastern part of the nation.

The seizure of the airport posed the first immediate challenge to Poroshenko, coming only a day after his overwhelming victory at the polls. Judging by his words and the government's military response at the Donetsk airport, he intends to take a more aggressive tack against the pro-Russian fighters.

“I am not going to hold any dialogues with the criminals. You don’t talk to terrorists,” he said at a press conference on Monday. “The anti-terrorist operation will not and cannot last for months, it will last just for hours.”

The Ukraine government also clearly recognizes the strategic importance of the airport. It was only a few months ago that pro-Russian fighters seized the main airport in Simferapol, Crimea, the first step in Russia's eventual takeover of that small part of Ukraine. Russia subsequently used the airport to send in reinforcements.

A pro-Russian militant at the airport in Donetsk, Ukraine, on Monday. Image: Alexander Khudoteply/AFP/Getty Images

The standoff at the Donetsk airport began at 3 a.m. local time on Monday when separatists stormed the terminal, saying they wanted Ukrainian troops to leave both the airport and the region. Three buses carrying more separatists arrived at 10 a.m. to reinforce the first group.

Separatists seized the Donetsk airport on Monday, halting flights and demanding that Ukrainian troops leave. Image: Alexander Khudoteply/AFP/Getty Images

By 2 p.m., bursts of gunfire and explosions could be heard for at least 30 minutes. At least three Ukrainian helicopters were flying overhead, along with two fighter jets that were releasing anti-missile flares. One helicopter fired at and destroyed a rebel anti-aircraft gun, according to Vladislav Seleznev, a Ukraine military spokesperson.

Seleznev said the military had launched an airstrike against the separatists after they rejected an ultimatum, the Associated Press reported.

Around 3 p.m., Ukrainian forces went on the offensive. Bursts of gunfire punctuated by mortar and RPG blasts echoed through the area. Government forces pushed the militia south into a residential area less than a quarter mile from the city's central train station.

Rebels and Ukrainian troops exchanged gunfire from their positions behind trees and bus stops.

At least one rebel was injured and carried to a car and driven away. One civilian - a parking attendant - was killed by stray bullet. The person's identity was unknown.

Smoke was seen rising from the Donetsk airport on Monday.

By nightfall, fighting was continuing, according to Reuters. Separatist leader Denis Pushilin said, "It's a full-blown military standoff."

Pushilin earlier declared martial law in the Donetsk region at midnight on Monday, vowing to push the Ukrainian military out by brute force should they not leave voluntarily.

The clash in Donetsk developed mere hours after Poroshenko claimed victory as Ukraine's president, becoming the first elected leader of the nation since Viktor Yanukovych was ousted in February. Poroshenko, a billionaire oligarch who visibly supported the Euromaidan uprising that led to Yanukovych's removal, won well over 50 percent of the vote; former Prime Minister Yulia Tymoshenko placed a distant second.

In his first post-election press conference, Poroshenko vowed both to intensify military efforts against armed separatists in the east and to hold peace talks with Russia as well, according to the Kyiv Post news site.

His election brought conciliatory words from Moscow as Russian Foreign Minister Sergei Lavrov said, "We are ready for dialogue with Kiev's representatives, with Petro Poroshenko."