Ukrainian President Volodymyr Zelensky on Thursday thanked European leaders for their military aid but warned that delays in sending fighter jets and long-range missiles could prolong the war.

Zelensky addressed a summit of his EU counterparts via video link from a Ukrainian train after visiting war-ravaged areas along the front line with Russian forces.

The secure connection was briefly lost once, a European official told reporters at the summit.

"The delay in supplying Ukraine with long-range missiles is already significant ... We cannot delay the transfer of weapons to our soldiers, who can protect Ukrainians from terror," Zelensky said according to a readout and video released by the presidency. 

Zelensky said he was "grateful" to Poland and Slovakia for pledging Soviet-era MiG-29 fighter jets to Ukraine, but said "we need more modern aircraft".

Zelensky also urged the leaders to impose more sanctions on Moscow, speed up work on Ukraine's EU membership bid and increase support for a peace plan proposed by Kyiv. 

"Ukraine is ready for a decision on the start of accession negotiations on EU membership already this year. The same readiness is needed from all of you -- every leader in Europe," he said. 

Zelensky said he had identified "five reasons that are prolonging the war and pushing peace further away". 

Kyiv has repeatedly pleaded with his Western allies to send it modern warplanes and long-range missiles to help push back Moscow's forces. 

As the summit began, Slovakia said it had transferred four MiG-29 jets to Ukraine, after becoming the first country alongside Poland to commit to sending older planes.

But Kyiv's backers have so far refused to provide Western-made aircraft. 

Before his call to the EU summit, Zelensky had visited the southern Ukrainian region of Kherson, partially controlled by Russian forces.  

Ukrainian forces recaptured Kherson city, the administrative centre of the region, in November following the withdrawal of Russian forces.