The White House has called a Russian convoy's move into Ukraine a "flagrant violation" of the country's sovereignty and urged Russia to remove it or face more economic sanctions.
White House deputy national security Ben Rhodes told reporters that sending the convoy into Ukraine adds to the escalation of the conflict "in a dangerous way."
The United States plans to discuss the situation with the UN Security Council later today, he added.
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Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.