NATO head Jens Stoltenberg said Wednesday Russia had recently increased the flow of heavy arms and equipment into eastern Ukraine but he declined to comment on Kiev's charge it has 9,000 troops on the ground helping pro-Moscow rebels.

"We will not go into specific figures or numbers," Stoltenberg said when asked about Ukrainian President Petro Poroshenko's remarks that more than 9,000 Russian soldiers were now in the separatist east.

Russia had been moving forces and equipment back and forth repeatedly but recently "we have seen an increase in Russian equipment inside eastern Ukraine... like tanks, artillery, armoured vehicles and advanced air defence systems," Stoltenberg said.

"This Russian military presence... does not contribute to a peaceful and negotiated solution," he said, calling on Russia once again to fully implement the Minsk peace accords it signed up to in September.

Poroshenko said at the World Economic Forum in Davos that there were "more than 9,000 troops of the Russian Federation on my territory, including more than 500 tanks and heavy artillery and armed personnel carriers".

The solution to the conflict "is very simple," he said, citing provisions in the Minsk accords.

"Stop supplying weapons. Stop supplying ammunition. Withdraw the troops and close the border. A very simple peace plan," Poroshenko said.