"Ukrainian bravery and western support is holding back their advance, but we cannot be complacent," he added.
"Ukraine needs our unwavering support to fight and win."
It comes as the Ministry of Defence said that Russian casualties, reported by the Ukrainian General Staff, were running at an average of 931 per day in November. The deadliest month for Russia had previously been March 2023, where Putin's army saw an average 776 losses each day during the height of the battle for Bakhmut."Although Defence Intelligence cannot verify the methodology, taken as a total including both killed and wounded, the figures are plausible," the Ministry of Defence said. "The last six weeks have likely seen some of the highest Russian casualty rates of the war so far.
"The heavy losses have largely been caused by Russia’s offensive against the Donbas town of Avdiivka."
Russia has continued its attempts to advance near Avdiivka, the eastern town that has been a Ukrainian stronghold and seen fierce fighting since the early days of the war, according to analysis by the Institute for the Study of War.
The Ukrainian General Staff said on Sunday that Kyiv's forces had beaten back Russian assaults as Moscow's troops strain to encircle the city. Meanwhile, Jens Stoltenberg said on Monday that Ukraine will become a member of Nato, subject to reforms, when the war is over.
The secretary general said all allies agree Nato will join the military alliance.
However, he acknowledged that full membership would be impossible while the war is ongoing.
This month Germany and the Netherlands pledged €10bn for Ukraine, he added.
Mr Stoltenberg also said that Sweden’s pending Nato membership “will make us all safer".