Story highlights Two humanitarian workers killed

Two Saudi border guards killed in exchange with rebels, Saudi Press Agency reports

Saudi forces in non-combat roles "coordinating and guiding" fight against Houthis, source says

(CNN) Saudi special forces assisted Yemeni fighters targeting Houthis and their allies in Aden, a Saudi source told CNN.

The special forces were on the ground in noncombat roles "coordinating and guiding" the battle against Houthi forces and fighters loyal to former Yemeni President Ali Abdullah Saleh, the source said. They also have helped parachute in weapons and communications equipment.

Houthi rebels, meanwhile, withdrew from the presidential palace and other key parts of the southern port city of Aden following heavy Saudi airstrikes, according to the source.

The withdrawal signaled a reversal from rebel gains the previous day, when Houthi forces seized the presidential palace. President Abdu Rabu Mansour Hadi, who fled the country last month, had stayed for a period of time after Houthis secured control of the capital, Sanaa, in January.

#Saudi tanks and heavy armor moves to #Yemen border - tense - stepping up security border guards in front Army behind pic.twitter.com/q6hDz0UwGZ — Nic Robertson (@NicRobertsonCNN) April 3, 2015

Houthis had controlled most large districts in Aden amid ongoing clashes.

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