Pro-Russia militia men arrive to take positions outside an airport, in Donetsk, Ukraine, on May 26. / Max Vetrov, AP
The Organization for Security and Cooperation in Europe said Tuesday it had lost contact with a four-person monitoring team last heard from outside Donetsk, the site of fierce battles between Ukrainian troops and insurgents.
In April, pro-Russia insurgents took a similar OSCE team hostage and held them for a week.
Reuters reports that the four -- an Estonian, a Swiss, a Turk, and a Dane -- encountered a road checkpoint on Monday evening and have not been heard form since.
The OSCE has had monitors in the country for weeks observing the military situation and also joining hundreds of other international groups to observe Sunday's presidential election.
The disappearance came as Ukrainian anti-terrorist forces launched a major attack against rebels who had taken over Donetsk International Airport on Monday.
Gunmen, believed to be pro-Russian, also stormed and set fire to a main ice hockey arena in Donetsk on Tuesday while border guards farther east reported battles with insurgents trying to bring weapons into Ukraine from Russia.
The Druzhba arena, home to the Donbass club, had been earmarked to host several games during the 2015 world hockey championships.
The attacks were the latest in an upsurge in fighting as Ukrainian anti-terrorist troops mount a major campaign against rebels in the wake of the weekend election of a new Ukrainian president, candy magnate Petro Poroshenko, who has vowed to negotiate a peaceful end to the crisis.
After a day-long battle at Donetsk International Airport, Ukraine Interior Minister Arsen Avakov said his forces were in control of the facility, Ukrinform reports.
"The airport is fully controlled. The enemy incurred serious losses, we have no losses," Avakov said in a statement issued by the ministry's press service. "The operation continued in the morning."
In the neighboring Luhansk region, also a heavily ethnic Russian area, the Ukrainian Border Guard Service said that its officers engaged in a gunbattle with a group of gunmen trying to break through the border from Russia. It said one intruder was wounded and the border guards seized several vehicles loaded with Kalashnikov assault rifles, rocket grenade launchers and explosives.
The press service of the State Border Guard Service of Ukraine said in a statement that up to 40 trucks carrying armed men are concentrated on the Russian side of the border, as far back as 6 miles, ready to enter Ukraine, the Kiev Post reported.
The Kiev Post, citing a Facebook post by Pavlo Hubarev, who refers to himself as the "people's governor of the Donetsk People's Republic," reported that 35 militants loyal to Moscow were killed when a truck they were traveling in was hit by a grenade.
Many of the killed Russian-backed fighters were riding together in a truck that was hit Monday either by artillery or rocket-propelled grenades from Ukrainian army helicopters, the newspaper said. The truck was destroyed.
Vladyslav Seleznyov, spokesman for the Ukrainian anti-terrorist service, confirmed that many armed "terrorists" were killed, but would not provide an official number. However, the Donetsk mayor confirmed at least 40 deaths and Kiev Post said its reporters counted even more.
Seleznyov confirmed the presence of foreign fighters, many from the Caucasus region of Russia.
Contributing: Associated Press
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