(CNN) The opposing sides in Yemen's conflict began direct talks in Sweden on Thursday as a step towards calming violence and averting further humanitarian crisis in the war-torn country.

However, the first direct discussions between the parties in two years started just hours after they traded threats.

Houthi rebels said they would completely shut down Sanaa airport -- which they control -- unless Saudi Arabia eased its air blockade.

Meanwhile, Saudi-backed Yemeni officials demanded a Houthi withdrawal from the strategic port city of Hodeidah.

United Nations Special Envoy Martin Griffiths said the discussions would assess "if there is an appetite for opening the Sanaa airport," and "how the two parties can contribute to a coherent economic plan."

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