UNITED NATIONS, May 9 (Xinhua) -- UN Secretary-General Antonio Guterres on Wednesday voiced deep concern over the recent and sharp escalation in the Yemen conflict, between Saudi Arabia and Houthi rebels, said his spokesman Stephane Dujarric.

Airstrikes by the Saudi-led military coalition hit a government building in the rebel-held Yemeni capital of Sanaa on Monday, and the Houthi rebels fired ballistic missiles on Sunday and Wednesday toward different targets in Saudi Arabia, including the capital city of Riyadh.

The secretary-general reminded all parties to the conflict that they must uphold international humanitarian law, including taking steps to protect civilians, said Dujarric in a statement. "All potential violations of international humanitarian law should be thoroughly investigated and those responsible for violations must be held accountable."

The UN chief appealed to the parties to refrain from further escalation. He reminded the parties that a negotiated political settlement through inclusive intra-Yemeni dialogue is the only way to end the conflict and address the ongoing humanitarian crisis, said the statement.

Yemen has been in civil war since 2015, pitting Houthi rebels and forces loyal to the government of Abdrabbuh Mansur Hadi. The Saudi-led military coalition is fighting the Houthis in a bid to restore the Hadi government.