Russia's military is intensifying its offensive to take full control of Ukraine's eastern region of Luhansk, while a Ukrainian official says the fighting will likely be prolonged due to the country's resistance.

Russian forces have captured most of the Luhansk region. To take full control, they are stepping up attacks on Ukraine's key stronghold cities of Severodonetsk and Lysychansk.

Russia's defense ministry said on Monday it destroyed Ukrainian military command posts, radar equipment and other targets near Lysychansk.

The battle has become fierce over the Donets River, which runs between Severodonetsk and Lysychansk.

Luhansk Governor Serhiy Haidai said two bridges over the river were destroyed by Russian attacks. He condemned Moscow because the destruction of the bridges has prevented the evacuation of civilians and the transport of relief supplies.

He also said Lysychansk has had the highest number of deaths in the region over the past several days.

In its analysis on Monday, the British defense ministry said, "Over the coming months, river crossing operations are likely to be amongst the most important determining factors in the course of the war."

A Ukraine defense ministry official said the Russian military has had to update its plan every 30 days since the start of the invasion and is carrying out a plan for 120 days of war.

The official also said the Russian military aims to take control of the Luhansk and Donetsk regions by late June.

He said Russia will continue attacking Ukraine because it has not yet achieved its main goals.