EU foreign policy chief Josep Borrell on Tuesday urged countries to balance fear of escalation against Ukraine's need to defend itself, saying Kyiv had the right to use Western arms to strike in Russia.

"According with the law of war, it is perfectly possible and there is no contradiction.

"I could retaliate or I could fight against the one who fights against me from his territory," Borrell said at the start of a meeting with European Union defence ministers. 

"You have to balance the risk of escalation and the need for Ukrainians to defend."

Ukraine has been pressing its Western backers to allow it to hit targets in Russia using the longer-range weaponry Kyiv's supporters are supplying. 

Key backers including the United States and Germany have been reluctant to permit Kyiv to strike over the border out of fear that it could drag them closer to direct conflict with Moscow.

Borrell's assertion was backed up by several other defence ministers at the gathering in Brussels. 

Dutch minister Kajsa Ollongren said The Netherlands had no restrictions and that she hoped "other countries that have different positions will change that".

"It is quite feasible that they have to strike also inside Russia," Ollongren said. 

"My perspective is that should not be the debate."

Estonia's Hanno Pevkur said he "truly hopes that all the countries who have these assets will also give the permission to Ukraine".

"It cannot be normal that Russia is attacking from very deep into Ukrainian territory and the Ukrainians are fighting with one hand behind their back," he said.

The calls came after NATO Secretary General Jens Stoltenberg said on Monday "the time had come" for allies to reconsider their restrictions, especially as Russia is pounding the Ukrainian city of Kharkiv from its own territory.

The debate over Ukrainian strikes inside Russia comes as NATO countries are struggling to come up with air defence systems desperately needed by Kyiv. 

Stoltenberg said last month that allies had agreed to give more air defences but since then there have been no new systems pledged. 

Dutch minister Ollongren said The Netherlands was now working on an initiative to assemble a Patriot air defence system in close cooperation with several partners to send to Ukraine. 

"We will deliver, from our own capabilities, several parts of the system so that we can assemble the whole system and deliver it to Ukraine," she said.