"I welcome allies contributing additional forces to NATO," the Western military alliance's Secretary-General Jens Stoltenberg said in a statement.
"NATO will continue to take all necessary measures to protect and defend all allies, including by reinforcing the eastern part of the alliance."
Britain said it was withdrawing some staff and dependants from its embassy in Ukraine in response to "a growing threat from Russia", a day after the United States said it was ordering diplomats' family members to leave.
US diplomats at the embassy in Kyiv were being allowed to leave voluntarily.