The head of the UK armed forces Admiral Tony Radkin said on Sunday that it was "unlawful and unhelpful" for Britons to go and fight against Russia in Ukraine, reports AFP.
His comments contradict British Foreign Secretary Liz Truss, who said she supported anyone who wanted to volunteer and follow an appeal for foreign fighters to come to Ukraine from President Volodymyr Zelensky.
"Support from the UK, support in whatever way you can. But this isn't really something that you want to rush to, in terms of the sound of gunfire."
Truss said last week that she would "absolutely" support Britons wanting to go to Ukraine to fight, and an unknown number have already gone.
"I think she was reflecting... that we can all understand that sentiment, and that sentiment needs to be channelled into support for Ukraine, but we're saying, as professional military people, that actually that is not necessarily the sensible thing to be doing," Radkin said.
Zelensky has urged foreigners to head to Ukrainian embassies worldwide to sign up for an "international brigade" of volunteers to help fight invading Russian forces.
He previously called on foreigners with combat experience to come to help defend his country, which has come under a withering Russian military assault from three sides since February 24.
The two heads of state spoke ahead of a diplomatic forum in Turkey on March 11-13 that Russian Foreign Minister Sergei Lavrov and his Ukrainian counterpart are due to attend. Turkey hopes to organise a meeting between the two.
"An urgent general ceasefire would make it easier to find a political solution and respond to humanitarian concerns," Erdogan said.
He urged Putin to allow the creation of "urgent" humanitarian corridors in Ukraine, saying he and the Russian leader could "open the way of peace together".
He told Putin that Ankara was "prepared to contribute in all possible ways towards a peaceful resolution (of the crisis)".
Official Turkish media said the conversation lasted an hour. Turkey is conducting a delicate balancing act.
On the one hand, it is a member of NATO and an ally of Ukraine. On the other, it needs to maintain good relations with Russia, on which it depends heavily for imports.
Erdogan has offered on several occasions to host talks between Kyiv and Moscow.