A senior US Defense Department official says that some of the Russian ground forces advancing toward Ukraine's capital Kyiv have been pushed back by Ukrainians.

The official said on Wednesday that one Russian unit that had been 20 to 30 kilometers northeast of central Kyiv is now about 55 kilometers away.

Another unit that's the closest to central Kyiv is "digging in" and setting up defensive positions about 15 to 20 kilometers northwest of the city, and is "not trying to advance right now."

The official added that the Ukrainians are not only defending well, but also mounting offensives in several areas and pushing the Russians back.

The official noted the Russians have launched more than 1,200 missiles. The eastern city of Mariupol has been heavily hit by long-range missiles, and some Russian ground troops are engaged in combat in the city.

Asked about Russia's possible use of biological or chemical weapons, the official noted that for now there are no signs their use may be imminent.

The official also denied there are any signs of Belarus moving its troops or preparing to join Russia's invasion of Ukraine.

The official explained that 800 million dollars' worth of additional military aid, including drones and anti-aircraft systems, that US President Joe Biden pledged last week should start arriving in Ukraine within a day or two.