Between 1,200 and 2,000 of the soldiers are to be organised into three battalion tactical groups, the Ministry of Defence (MoD) said.
It is “highly unlikely” that Vladimir Putin’s regime would make such a move unless it was forced to take desperate measures as a result of “unexpected losses” it sustained during its invasion of Ukraine, the department claimed.
Britain and its allies have agreed to send more weapons – such as longer range artillery, ammunition, and anti-aircraft weapons – to Ukraine, defence minister Ben Wallace said.
He told reporters on Thursday: “There’ll be more lethal aid going into Ukraine as a result of today. A number of countries have come forward either with new ideas or indeed more pledges of money.”
It comes days after Russia claimed it would scale-back its military operations to focus on the pro-Moscow separatist-controlled region of Donbas in eastern Ukraine.
But Nato general secretary Jens Stoltenberg said that Russian forces are regrouping and not withdrawing.
He told reporters in Brussels: “According to our intelligence, Russian units are not withdrawing but repositioning. Russia is trying to regroup, resupply and reinforce its offensive in the Donbas region.
“At the same time, Russia maintains pressure on Kyiv and other cities. So we can expect additional offensive actions, bringing even more suffering.”
The situation remained “very difficult” in the south of Ukraine and in the Donbas region, Volodymyr Zelensky said.
The Ukrainian president reiterated that Russia was building up forces near the besieged Black Sea port city of Mariupol.
“There will be battles ahead. We still need to go down a very difficult path to get everything we want,” Mr Zelensky said in a late-night video address to officials in Brussels.
A “humanitarian catastrophe” has been inflicted on Mariupol that has seen thousands of people killed, he added.
A total of 148 children have been killed by Russian forces’ shelling and airstrikes since the start of the invasion, Ukraine’s defence ministry said.
More than 10 million Ukrainians have fled their homes, it said in a statement. Reuters said it could not independently verify the claim.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies