Russia-Ukraine updates: Russian missiles hit close to nuclear reactors: IAEA director

Russian forces are advancing toward the city of Lysychansk in eastern Ukraine's Luhansk region, putting the Lysychansk-Severodonetsk pocket "under increasing pressure," the British Defense Ministry said. Some Ukrainian forces have withdrawn from Lysychansk, "probably to avoid being encircled," the officials said.

The Ukrainian army claims Ukrainian soldiers stopped the Russian offensive in the southern outskirts of Lysychansk on Thursday, inflicting losses and forcing them to retreat, and that to resume the offensive, Russian forces put forward reserves.

-ABC News' Fidel Pavlenko

The U.S. is expected Thursday to announce a new $450 million military aid package that will include more High Mobility Artillery Rocket Systems, known as HIMARS.

Ukrainian Minister of Defence Oleksii Reznikov tweeted Thursday that the initial HIMARS package from the U.S. has arrived in Ukraine, and he thanked Secretary of Defense Lloyd Austin for the "powerful tools."

The new aid package will include more small arms, grenade launchers, vehicles and patrol craft.

The U.S. has committed over $6 billion to Ukraine since Russia’s invasion began, according to the Pentagon.

-ABC News' Luis Martinez

More than 8 million people have fled Ukraine since the start of the Russian invasion on Feb. 24, according to an update by the United Nations.

More than 4 million Ukrainians fled through Poland -- by far the preferred route for the displaced, the U.N. report said. Hungary, the second most used route, reported just over 800,000 crossings.

-ABC News' Edward Szekeres, Max Uzol, Fidel Pavlenko and Yuriy Zaliznyak

The Belarusian Defense Ministry announced “mobilization exercises” on Wednesday in the Gomel region bordering Ukraine.

The military drills, scheduled to last until July 1, will include special operations forces as well as freshly called up conscripts, Belarusian officials said on Telegram.

The Belarusian army has already been placing wooden dummies of tanks on the Ukrainian border to demonstrate their presence, Ukrainian Ministry of Defense spokesperson Alexander Motuzyanyk said on Wednesday.

Real weapons are arriving near the Ukrainian border, too, with a new batch of Russian missiles for the S-300 anti-aircraft missile system being brought to Belarus on Tuesday night, local monitoring groups reported. The delivery included at least 16 missiles, with the likely addition of one Pantsir missile defense system, the report said.

Still, Ukrainian officials maintain that “at this stage of the war,” there is no imminent threat “of the Belarusian army invading” Ukraine, Kyrylo Budanov, head of the Main Intelligence Directorate, said on Wednesday as cited by local media.

-ABC News' Edward Szekeres, Max Uzol, Fidel Pavlenko and Yuriy Zaliznyak