Ukrainian forces continued to fight in the besieged city of Mariupol on Wednesday after a Russian ultimatum to troops holed up in the Azovstal steel mill to lay down their arms passed without a mass surrender.

Amid the siege in Mariupol and a shift in the fighting to eastern Ukraine, U.S. officials are considering a new military aid package for Ukraine

Reuters and the BBC reported there were no signs of Ukrainian forces giving in, and Oleksiy Arestovych, a senior adviser to Ukrainian President Volodymyr Zelenskyy, said in a social media post that the city was being held as fighting continued.

Remaining Ukrainian forces in Mariupol have held onto the sprawling steel plant for several days despite the Russian military dropping heavy bombs and threatening to flatten the building, according to the deputy commander of the forces inside. 

The plant covers 4 square miles, including a network of tunnels. More than 1,000 civilians and troops are inside, Reuters reported, while Russian estimates say a few thousand Ukrainian troops are within.

With humanitarian corridors established for the first time in several days, Mariupol’s mayor on Wednesday urged the city’s remaining civilians to flee.

Capturing the city is key to Russia’s goals, as it would secure a land corridor from Russia to the occupied territory of Crimea, and the steel mill complex is likely the last remaining Ukrainian stronghold.

Moscow’s attacks on Mariupol are part of its new wartime strategy: pivoting to a heavy offensive in the eastern Ukrainian region of Donbas, parts of which were controlled by Russian-backed forces prior to the invasion.

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►More than 5 million refugees have fled Ukraine since the start of the Russian invasion, according to a U.N. refugee agency’s latest tally as of Wednesday.