Russian forces claim they are about to seize the key Ukrainian city Bakhmut, and are calling on Ukraine's president to withdraw his troops.

The head of Russia's Wagner mercenary group, Yevgeny Prigozhin, said on social media on Friday that his units had "practically surrounded Bakhmut," a stronghold of Ukrainian forces in Donetsk, a region in the country's east.

He added, "Only one route is left" for the Ukrainian soldiers who remain.

In a video released the same day, Ukrainian President Volodymyr Zelenskyy expressed defiance.

He said repelling the enemy is a "prerequisite for the success of our defense operations in the coming months."

The president expressed his gratitude to Ukrainian soldiers defending Bakhmut for their bravery.

Zelenskyy attended a meeting in the western city of Lviv on Friday. Also present were the chief prosecutor of the International Criminal Court, Karim Khan, and other top legal officials from the United States, Britain and the European Union.

The Ukrainian president asked for further cooperation to hold Russia accountable for its crimes and human rights violations.

He said, "True justice is when all the people who are guilty are brought to justice according to the law."

In the United States, the Defense Department on Friday announced a new military aid package for Ukraine worth up to 400 million dollars.

The package includes additional ammunition for High Mobility Artillery Rocket Systems, as well as portable armored vehicle-launched bridges to move tanks and other vehicles across rivers.

The equipment is apparently aimed at enabling the prompt deployment of military vehicles supplied by Western countries.

The department says the United States and its allies will continue to work together to provide Ukraine with important military capabilities.