A US professor of international law said the US-led NATO's expansion is the root cause of the Russia-Ukraine conflict.

Francis A. Boyle, a professor of international law at the University of Illinois, made the remarks in an interview to China Daily's Pan Yixuan.

Boyle said former US secretary of state James Baker, during the tenure of then US president George H.W. Bush, as well as many then European NATO states' leaders made a solemn pledge before former Soviet Union leader Mikhail Gorbachev saying NATO would not expand to the east if the Soviet Union agreed to the two Germany's reunification. The promise is binding as a matter of international law, Boyle stressed.

However, according to Boyle, the continuous expansion of NATO has irked Russia, and the US-led NATO rejected Russia's objection, which made the situation deteriorate drastically.

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To help address the disaster in Ukraine, Boyle said the US should respond to Russia's concerns about a buffer zone and announce that NATO will not expand, and Ukraine will not join NATO as a member state.

Boyle also said that China could play a bigger role of mediator to ease the crisis.

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