Ukraine said on Monday its troops continued to drive rebels back in the east of the country, toward the border with neighboring Russia which has supported the separatist uprising.

The speed of the offensive appeared to have taken the rebel leadership by surprise. Alexander Borodai, the self-appointed prime minister of the breakaway Donetsk People’s Republic, reportedly left Ukraine for Moscow. Ukrainian media reported that rebel commander Igor Bezler fled the city of Horlivka on Sunday as it was attacked by government forces.

At an improvised news conference on Monday, rebel commander Igor Girkin, also known as Strelkov, said the Ukrainian military had used “an unexpected amount of artillery.” He claimed the army had deployed “mercenaries” to fight his rebels, some “of the negroid race.”

Authorities in Kiev said soldiers retook the towns of Torez and Shakhtarsk from the separatists while fighting was in progress in the area where a commercial airliner was shot down two weeks ago. Ukrainian army units reconquered Sievierodonetsk, a city north of the rebel stronghold of Luhansk, on Sunday.

Another rebel leader, Vladimir Antyufeyev, told reporters in Donetsk that his fighters had attempted to escort foreign forensic experts to the plane’s crash site but encountered fighting and turned back.

Investigators from the Netherlands, which lost 193 nationals when a Malaysia Airlines jet was brought down over the conflict area, said they where unable to reach the crash site on Sunday.

After consulting with the European country’s foreign minister, Frans Timmermans, on Monday, President Petro Poroshenko promised not to order Ukrainian army operations within a radius of twenty kilometers from where the plane came down.

Western countries believe the rebels shot down the passenger jet, mistaking it for an Ukrainian military transport plane, with missiles that were supplied by Russia. Russia denies it gave the separatists any weapons — yet Russian weapons have found their way into eastern Ukraine.

Antyufeyev himself is a Russian citizen and was named deputy prime minister of the Donetsk republic after Borodai briefly visited Moscow earlier this month.

A former Soviet special police officer, Antyufeyev was involved in an attempt to overthrow the government of Latvia when the Soviet Union collapsed in 1991. He then took control of the security forces of Moldova’s breakaway region of Transnistria, which is home to a large ethnic Russian minority, a position he held until 2012.

After Russia annexed Ukraine’s Black Sea peninsula of Crimea in March, lawmakers in Transnistria asked to join the Russian Federation as well.

The separatists in the Donetsk and Luhansk regions of Ukraine similarly seek Russian annexation. Russia has stopped short of endorsing their attempts to secede from Ukraine and proposed turning the country into a federation instead. Ukraine’s government sees this as a ploy to permanently divide the country between the regions in the west that tend to favor closer relations with the rest of Europe and the largely Russian-speaking southeast.

On Sunday, the United States accused Russia of firing across the border at the Ukrainian military. Videos purporting to show Russian armored personnel carriers, infantry fighting vehicles and missile launchers moving across the frontier were uploaded to YouTube the same day.