Ukrainian President Volodymyr Zelensky on Wednesday warned that any delays in Western aid to Kyiv emboldened the Kremlin, almost two years into Russia's invasion.

Zelensky was in Lithuania's capital Vilnius as part of a surprise visit to one of Kyiv's staunchest allies.

"He (Russian President Vladimir Putin) is not going to stop," he said at a news conference with his Lithuanian counterpart Gitanas Nauseda. "He wants to occupy us completely.

"And sometimes, the insecurity of partners regarding financial and military aid to Ukraine only increases Russia's courage and strength," he added.

He was speaking as other Western countries are wavering over continued support for Ukraine.

Zelensky warned that Putin would not stop at Ukraine but would attack other neighbours unless allies joined forces to stop him.

"He (Putin) won't finish this (war), until we all finish him together," the Ukrainian leader said.

"We must understand that Lithuania, Latvia, Estonia, Moldova may be next if we (Ukraine) do not withstand (Russia)," he added.

His comments came as Ukraine is facing renewed aerial assaults from Russia, prompting pleas for more support to Kyiv, which is "sorely lacking" modern air-defence systems.

"In recent days, Russia hit Ukraine with a total of 500 devices: we destroyed 70 percent of them," Zelensky said.

"Air-defence systems are (the) number one (thing) that we lack," he added.

Zelensky's visit to Lithuania comes as he seeks to bolster wavering support among other Western backers.

The tour of the three former Soviet republics -- all now EU and NATO members -- marks Zelensky's first official trips abroad this year.