In another development, Russian Defence Minister Sergei Shoigu has ordered generals to focus on destroying Ukraine's long-range missile and artillery weapons.
The defense ministry said the weapons were being used to shell residential areas of eastern Ukraine under the control of Russian-backed separatists, and to set fire to wheat fields and grain storage silos.
The ministry also said that a Russian missilestrike on Sunday on the Donetsk village of Kostiantynivka had killed up to 250 mercenaries fighting on the Ukrainian side, as well as destroyed 19 military vehicles.
The claims could not be independently verified.
In recent weeks, the US has supplied Ukraine with long-range missiles, including precision-guided High Mobility Artillery Rocket Systems (Himars), which Kyiv says are beginning to make a difference on the battlefield.
Himes is far more accurate than Russia's equivalent systems and has been credited with a string of recent attacks deep into Russian-held territory.
In another fighting, Ukraine's southern command says two Russian missiles have hit a military target outside the southern port of Odesa, while another has struck a bridge across the Dniester estuary.
A fourth was shot down over the Black Sea. Officials say there are no casualties and the fire at the military facility is under control.
They say the bridge has been out of action since Russian forces began targeting it in April.
Meanwhile, British military intelligence has said Russia is using mercenaries from the Wagner group to reinforce the front lines in Ukraine.
The Ministry of Defence (MoD) in London said Wagner had been active in recent fighting in the Luhansk region, which had inflicted heavy casualties on the Russian paramilitary organization.