Russian forces defeated a team of Ukrainian special forces which were rushed to the embattled eastern city of Pokrovsk in an attempt to prevent Russian forces from advancing further into the city, the Russian defence ministry said.

Russia wants to take the whole of the Donbas region, which comprises Luhansk and Donetsk provinces. Ukraine still controls about 10% of Donbas - an area of about 5,000 square km (1,930 square miles) in western Donetsk.

Capturing Pokrovsk, dubbed "the gateway to Donetsk" by Russian media, and Kostiantynivka to its northeast which Russian forces are also trying to envelop, would give Moscow a platform to drive north towards the two biggest remaining Ukrainian-controlled cities in Donetsk - Kramatorsk and Sloviansk.

Ukraine said that it had deployed special forces in Pokrovsk earlier in the week, landing a Black Hawk helicopter. At least 10 Ukrainian servicemen could be seen dismounting from a helicopter in a field in a video seen by Reuters.

Russia's defence ministry said it had killed the group of Ukrainian military intelligence special forces.

"All 11 people who disembarked from the helicopter were killed," the ministry said, adding that Russian forces were pushing forward in Pokrovsk and clearing the area.

Russian forces, it said, had pushed back an attempt by Ukraine's 425th Separate Assault Regiment "Skala" to break out of Hryshyne, northwest of Pokrovsk, while the pincer around Ukrainian forces in the area was closing.

Battles around Hryshyne are significant, as they indicate Russian forces are close to cutting off Ukrainian supply lines to Pokrovsk.

Reuters was unable to verify battlefield reports from either side due to reporting restrictions imposed by both sides.

Ukraine's only mine producing coking coal - used in its once vast steel industry - is around six miles (10 km) west of Pokrovsk. Ukrainian steelmaker Metinvest said in January it had suspended mining operations there.

Russian President Vladimir Putin says Donbas is now legally part of Russia but Kyiv and the West reject Moscow's seizure of the territory as an illegal land grab.

Russian forces used a pincer movement to almost fully encircle Pokrovsk and threaten Ukrainian supply lines, then harried Ukrainian forces by sending in small units and drones to disrupt logistics and sow chaos in their rear.

Essentially, Russia's tactics carved what Russian military bloggers called a grey zone of ambiguity out of the city where neither side had control but which was extremely difficult - and costly - to defend.

To clear both Pokrovsk and nearby Myrnohrad may take some time and so delay Russia's formal announcement.