Several NATO countries, including the US, Germany and the UK, have ruled out deploying ground troops to Ukraine, after French President Emmanuel Macron said "nothing should be excluded".
Mr. Macron said there was "no consensus" on sending Western soldiers to Ukraine.
Kremlin spokesperson Dmitry Peskov has warned of direct conflict if NATO troops are deployed there.
Russian forces have recently made gains in Ukraine and Kyiv has urgently appealed for more weapons.
Mr. Macron told a news conference on Monday evening that NATO should not rule out a need for security that justifies some elements of deployment.
The French leader was speaking in Paris, which is hosting a crisis meeting in support of Ukraine, attended by heads of European states, as well as the US and Canada.