KYIV, Ukraine, June 26, 2025 (BSS/AFP) - Ukraine's army chief on Thursday ordered defensive lines to be built more quickly in the northeastern Sumy region, where Russian forces have advanced over recent months.

Sumy lies over the border from Russia's Kursk region where Ukrainian forces launched an audacious landgrab last year that Moscow took months to push back, with the help of North Korean forces.

Kyiv says Russia, which invaded Ukraine more than three years ago, has now amassed 50,000 troops with the goal of advancing deeper into the Sumy region.

"Work is ongoing, but it needs to be accelerated, given the demands of modern warfare," Ukraine commander-in-chief Oleksandr Syrsky said, following a working trip to Sumy where he met with military officials.

Syrsky said "anti-drone corridors" were needed to protect Ukrainian troops and logistics routes. The speed at which this work was being carried out "must be significantly increased", he added.

Russian President Vladimir Putin said this month his military could try to capture Sumy city, the regional capital some 30 kilometres (18 miles) from the border.

Russian troops are steadily advancing across the sprawling front line in Ukraine, but at significant cost.