Ukrainian President Volodymyr Zelenskyy has renewed his resolve to continue fighting against invading Russian forces with support from Western nations, after losing control of the key eastern city of Avdiivka.

In a video message uploaded to the president's website on Sunday, Zelenskyy called the past week "very challenging."

He said, "The hearts of all those who care have experienced feelings of concern for our heroes, for our Avdiivka" and "for the fate of our entire state."

The message was posted a day after the Ukrainian military said it will withdraw from the city in the Donetsk region. Russia's defense ministry says its forces have taken full control of Avdiivka.

Zelenskyy said in the message that attending last week's Munich Security Conference affirmed that the "Ukrainian perspective on the global agenda was supported by our partners."

He also mentioned that Ukraine has signed long-term security agreements with Germany and France.

The president concluded the message by saying that "We must achieve our Ukrainian goals in this war."

Meanwhile, Kremlin spokesperson Dmitry Peskov told Russia's state-run Tass news agency that the country's troops are continuing their offensive in Avdiivka without allowing Ukrainian counterparts to build new positions.

Russian forces are believed to be intensifying efforts to take control of the entire Donetsk region.

US think tank the Institute for the Study of War said on Sunday, "Ukrainian forces will likely be able to establish new defensive lines not far beyond Avdiivka, which will likely prompt the culmination of the Russian offensive in this area."