KYIV: Ukraine said on Friday it was building a reinforced concrete wall and other fortifications on its border with Belarus, a close Kremlin ally that Moscow used as a staging ground for its Feb. 24 invasion of Ukraine.

Presidential adviser Kyrylo Tymoshenko said a 3 km barbed-wire-topped wall had gone up in the region of Volyn on the border with Belarus, which Kyiv says remains a threat. There were also sandbags and trenches, he said.

"That is not the end of it, but we are not going to disclose details," the official said.

He added that other work was also under way in the regions of Rivne and Zhytomyr that also border Belarus, but he did not elaborate.

First Russian soldiers arrive in Belarus for joint force

He published photographs of security officers uncoiling barbed wire next to a wall that appeared more than two metres high.

Ukraine's defence minister told Reuters on Thursday that there remained a threat from Belarus, but that forces there were "in a passive position". Ukraine's border with Belarus is more than 1,000 kms long.