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The kingdom's security forces were allegedly killed when rebel shelling hit military sites in Saudi Arabia's Najran, Jizan and Asser provinces, a rebel official told Yemen's Saba news agency.

Saudi Arabia's interior ministry on Thursday announced the death of just one soldier who it said was killed on a border post in south Dhahran, contradicting higher death toll claimed by the Houthis.

In Yemen's Marib, five fighters were killed in clashes across the province, an official said, according to state news.

The clashes come just a day after humanitarian groups demanded access to civilians on the brink of famine in Yemen, describing the two-year civil war in the Arab world's poorest state as an "open-air massacre".

The Saudi-led coalition's war against Houthi rebels in Yemen has cost more than 10,000 lives since March 2015 and brought the country to the brink of famine.

Sunday will mark two years since the coalition launched its bombing campaign in Yemen.