The city of Sievierodonetsk, the administrative centre of the Luhansk region, has faced relentless Russian bombardment. Ukrainian troops fought the Russians in house-to-house battles before retreating to the huge Azot chemical factory on the city's edge, where they remain holed up in its sprawling underground structures in which about 500 civilians also found refuge.

In recent days, Russian forces have made gains around Sievierodonetsk and the neighbouring city of Lysychansk, on a steep bank across the river, in a bid to encircle Ukrainian forces.

Sievierodonetsk and Lysychansk have been the focal point of the Russian offensive aimed at capturing all of the Donbas and destroying the Ukrainian military defending it, the most capable and battle-hardened segment of the country's armed forces.

The two cities and the surrounding areas are the last major pocket of Ukrainian resistance in the Luhansk region, 95% of which is now under the control of Russian troops and local separatist forces. The Russians and separatists also control about half of the Donetsk region, the second province that is part of the Donbas.

Luhansk Gov. Serhiy Haidai said that the Ukrainian troops have been given the order to leave Sievierodonetsk to prevent bigger losses.

``Regrettably, we will have to pull our troops out of Sievierodonetsk,'' Haidai told The Associated Press. ``It makes no sense to stay at the destroyed positions, and the number of killed in action has been growing.''

He said that the Ukrainian soldiers have ``received the order to retreat from Sievierodonetsk to new positions in fortified areas and continue resistance from there.''

Haidai noted that Ukrainian troops still remain in Sievierodonetsk, facing a massive Russian bombardment that has destroyed 80% of buildings.

``As of today, the resistance in Sievierodonetsk is continuing,'' Haidai told the AP. ``The Russians are relentlessly shelling the Ukrainian positions, burning everything out.''

Haidai said the Russians were also advancing toward Lysychansk from Zolote and Toshkivka, adding that Russian reconnaissance units conducted forays on the city edges but were driven out by its defenders.

The governor added that a bridge on a highway leading to Lysychansk was badly damaged in a Russian airstrike and became unusable for trucks.

The Russian Defense Ministry declared Friday that four Ukrainian battalions and a unit of ``foreign mercenaries'' totalling about 2,000 soldiers have been ``fully blocked'' near Hirske and Zolote, south of Lysychansk. The claim couldn't be independently verified.

Following a botched attempt to capture Kyiv, Ukraine's capital, in the early stage of the invasion that started Feb. 24, Russian forces have shifted focus in the war to the Donbas region, where the Ukrainian forces have fought Moscow-backed separatists since 2014.

After repeated requests to its Western allies for heavier weaponry to counter Russia's edge in firepower, Ukrainian Defense Minister Oleksii Reznikov said a response had arrived in the form of medium-range American rocket launchers.

A U.S. defence official confirmed Wednesday that all four of the promised High Mobility Artillery Rocket Systems, or HIMARS, were in the hands of Ukrainian forces but said it wasn't clear if they have been used yet.

The U.S. approved providing the precision-guided systems at the end of May, and once they were in the region, Ukraine's forces needed about three weeks of training to operate them. The rockets can travel about 45 miles (70 kilometers).

The U.S. will send an additional $450 million in military aid to Ukraine, including four more medium-range rocket systems, ammunition and other supplies, U.S. officials announced Thursday.