By Charlotte Cook, Hamish Cardwell and Ben Strang of RNZ

The Government does not know where the body of a New Zealand soldier killed in Ukraine is, including whether it is being held by Russians.

Corporal Dominic Abelen died fighting Russian forces while on leave without pay and was not on active duty.

Multiple sources have told RNZ they believe his body is in Russian hands but a spokesperson for Prime Minister Jacinda Ardern's office said they simply did not know whether that was true or not.

Video also shows Russian soldiers removing items from dead people in a trench.

The Ministry of Foreign Affairs and Trade said it did not comment on consular cases for privacy reasons.

The Defence Force would not comment and is also refusing to say if it is investigating whether other New Zealand soldiers are fighting in Ukraine.

Corporal Abelen had not been given permission to enter Ukraine and he had not told them about his plans, the NZDF said in a statement.

"It is not known how many NZDF personnel on leave without pay may be in Ukraine. No personnel have been approved by NZDF to enter the country," the statement said.

It declined to say whether it was checking the whereabouts of all its soldiers on leave, including without pay.

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Ex-soldier and former mayor of Tauranga Tenby Powell is in Ukraine providing humanitarian aid and helping with the evacuation of civilians.

He was working closely with the Defence Force to bring back the body, he said.

"We need to bring our boy back and reunite him with his family and his country, absolutely, but there's an awful lot that needs to go on before that including getting to him and extracting him from where he is.

"I sincerely hope ... through diplomatic channels, and through other operational channels, [we] find a way to bring him home as soon as possible."

'Hero of the Ukrainian people' - ambassador

Ukraine's ambassador to New Zealand Vasyl Myroshnychenko said Abelen was a hero to the Ukrainian people.

He said his heart went out to Abelen's family.

"It's really tragic, he died in action courageously defending human rights, defending freedom, defending democracy in Europe.

Corporal Dominic Abelen, who was killed in Ukraine while on unpaid leave from the New Zealand Defence Force. Photo / NZDF, RNZ

"He is a hero of the Ukrainian people, who will remember him."

New Zealand should send more support to Ukraine, Myroshnychenko said.

The Russian Embassy in New Zealand reiterated a previous statement from the Russian Foreign Ministry that it was not responsible for the lives of foreign citizens who took part in "illegal military activities in Ukraine".

Possible penalties for foreign fighting

Barrister Christopher Griggs, who served in the navy for 25 years as a legal adviser, said New Zealand soldiers contemplating fighting for Ukraine should know it was a major breach of the rules, possibly punishable by jail.

It was Cabinet's decision where soldiers were deployed and those in uniform must do what they are told, he said.

"If a member of the Defence Force just doesn't accept that and goes anyway, then that really is direct defiance of the will of the government.

"It certainly could be viewed as a serious breach of discipline."

Possible penalties included up to two years in military prison or being removed from the Defence Force.

The Defence Force would never give a soldier permission to fight for another country, he said.

Former soldier Aaron Wood said Abelen would have known that by going to Ukraine he risked losing his job, or going to military prison.

Wood said that Abelen went anyway and showed the strength of his convictions.

Former Defence Minister Ron Mark, who recently visited Ukraine, said there were only a small number of New Zealanders fighting there, and this was the first instance of a current NZDF soldier travelling to Ukraine while on leave from the army.

- RNZ