Washington: Russia`s border with eastern Ukraine has softened to the point of becoming completely porous, while an internal Ukrainian dividing line hardens, NATO`s top general said on Monday, cautioning about the risk of another "frozen conflict."

U.S. Air Force General Philip Breedlove, NATO`s supreme allied commander Europe, said he believed there were about 250 to 300 Russian troops inside eastern Ukraine helping train and equip Russian-backed forces.

Some seven Russian battalion task groups are across the international border in Russia, Breedlove said, adding that the composition of those forces made them difficult to quantify.

The international border had faded from view as attention shifts to the demarcation line separating pro-Russian militias and Ukrainian forces.

"We have an international border. That is the border between Russia and Ukraine. That should be the border that we’re trying to close, the border that we’re trying to enforce," he said.

"The focus seems to be now on the control of this line of demarcation and patrolling the line of demarcation."

Asked about his concerns, Breedlove said: "I`m concerned that the conditions are there that could create … a frozen conflict."

Breedlove`s comments came after elections in breakaway regions of Ukraine over the weekend that further deepened the standoff with Russia over the future of the former Soviet state.

Ukraine lost control in March of its Crimean Peninsula, which was annexed by Russia.

Breedlove said he had no indications Russia has moved tactical nuclear weapons into Crimea, as reported by one U.S. publication. He said Russia has publicly spoken about stationing aircraft there in the future that could carry out a full range of missions, but acknowledged: "I have no indication of what they`re actually going to do."

More than 4,000 people have been killed in the Ukraine conflict, which has led to U.S. and European Union sanctions against Russia.

NATO leaders have said the alliance has no plans to intervene militarily in Ukraine, which is not a NATO member. But they have beefed up NATO`s eastern defenses to reassure nervous allies and NATO has suspended practical cooperation with Russia.