"We've seen a pretty good withdrawal of the Russian forces from inside Ukraine but, make no mistake, there remain Russian forces inside eastern Ukraine," U.S. Air Force General Philip Breedlove told reporters at NATO's military headquarters near Mons in Belgium.

Some Russian troops stationed near the Ukraine border had left and others appeared to be preparing to leave.

"But the force that remains and shows no indications of leaving is still a very, very capable force," he said.

He spoke before Sunday's parliamentary election in Ukraine in which President Petro Poroshenko seeks a mandate to press ahead with his plan to end a separatist conflict and pursue integration with mainstream Europe.

NATO Secretary-General Jens Stoltenberg, making his first visit to NATO's military headquarters since he took over at the start of this month, said Russia remained in violation of international law in Ukraine.

"They are still violating the sovereignty and the territorial integrity of Ukraine by having Russian forces in Ukraine," the former Norwegian prime minister told reporters while visiting the NATO operations centre that monitors crises in Ukraine and elsewhere.

Russia was also destabilizing the situation in Ukraine by keeping forces on the border, he said, after being shown round by Breedlove.

NATO has suspended practical cooperation with Russia in protest against Moscow's annexation of Crimea and its support for the pro-Russian separatists in eastern Ukraine.

NATO would continue to call on Russia to withdraw its forces both from inside Ukraine and from the border and to use all its influence to make sure that the ceasefire in eastern Ukraine was respected, Stoltenberg said.

"We need to find a political solution to the challenges we see in Ukraine and a pre-condition for that is of course to have an effective ceasefire," he said.