Ukrainian President Volodymyr Zelensky inspected defensive fortifications in the northeastern Kharkiv region on Tuesday and issued his latest call for military aid as Russia intensified its aerial attacks on the area.

In a sign of the pressure Moscow is exerting on the region, Russia fired a guided bomb at the city of Kharkiv just as Zelensky announced his visit. It injured at least three people, the mayor said.

"Kharkiv region is a very important area. We have to be prepared. And the Russians must see that we are ready to defend ourselves," Zelensky said on Telegram.

He posted a video showing him talking to troops and walking through a trench as excavators dug up land.

Russia has bombarded Kharkiv, Ukraine's second city, since it first invaded in February 2022, and has escalated its attacks in recent weeks.

Ukraine is struggling to protect its cities from incoming Russian missiles, drones and bombs amid shortages of crucial ammunition and air defence systems.

Zelensky has been demanding on an almost daily basis that Kyiv's Western backers increase their supplies of arms.

"Kharkiv needs robust defence. There is a solution to Russia's constant terror. We need air defence systems and missiles for them," Zelensky had said earlier on Tuesday.

"The world has no right to remain indifferent while Russia deliberately destroys the city day after day and kills people in their homes," he said.

A $60 billion aid package has been held up in the US Congress since last year amid domestic political wrangling.

Zelensky said on Sunday that Ukraine will lose the war if Washington does not deliver sufficient aid.