A Russian military statement said it had broken through two fortified defence lines in Ukraine's east.

Bolstered by tens of thousands of reservists drafted in December, Russia has intensified attacks across southern and eastern Ukraine in recent weeks, and a major new offensive is widely expected as the first anniversary of its invasion nears.

READ MORE

China's Xi accepts invitation for state visit to Iran

US Secretary Antony Blinken to visit quake-hit Turkey on Sunday

"The enemy's offensive continues in the east, (with) round-the-clock attacks," Ukrainian Deputy Defence Minister Hanna Malyar said."

The situation is tense. But our fighters are not allowing the enemy to achieve their goals and are inflicting very serious losses," she wrote on the Telegram messaging app on Wednesday.

The Luhansk and Donetsk regions, Ukraine's industrial heartland, are now partially occupied by Russia.

Earlier, the Russian Defence Ministry said Ukrainian forces had retreated in the face of Russian operations in the Luhansk region, although it gave no details and Reuters was not able to verify this and other battlefield reports.

"During the offensive... the Ukrainian troops randomly retreated to a distance of up to 3 km (2 miles) from the previously occupied lines," the ministry said on Telegram.

"Even the more fortified second line of defence of the enemy could not hold the breakthrough of the Russian military."

The ministry did not specify in which part of Luhansk the offensive took place. The Luhansk and Donetsk regions make up the Donbas, Ukraine's industrial heartland, now partially occupied by Russia which wants full control.

In Kyiv, the capital's military administration said six Russian balloons that may have contained reconnaissance equipment were shot down after air raid sirens blared.

"The purpose of launching the balloons was possibly to detect and exhaust our air defences," it said on the Telegram messaging app. Russia did not immediately comment.

Reuters