( 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 . Houthis had controlled most large districts in Aden amid ongoing clashes . The latest action heats up an already intense conflict that could have ramifications across the Middle East . If the Saudi-led coalition of nine countries takes the fight to the ground in Yemen , the consequences could be severe . Houthis are battle-hardened guerrilla fighters and could cross into Saudi Arabia . They 've already threatened suicide bomb attacks inside Saudi Arabia . Yemen , already the home base to al Qaeda in the Arabian Peninsula , teeters on the brink of becoming a failed state and an even more fertile breeding ground for extremism . The United Nations , citing reports from humanitarian partners , said 519 people have been killed and nearly 1,700 others wounded in the past two weeks . Tens of thousands of people have fled their homes and sought refuge in Djibouti and Somalia , Valerie Amos , under-secretary-general and emergency relief coordinator , said in a statement . Two humanitarian workers were killed Friday in Yemen . The brothers working for the local branch of the Yemen Red Crescent Society were shot in Aden while evacuating wounded people to a waiting ambulance , the International Committee of the Red Cross reported . They were identified as Khaled Ahmed Bahuzaim and Mohammed Ahmed Bahuzaim . Two Saudi border guards were killed Friday after an exchange of fire with Houthi rebels along the kingdom 's frontier with Yemen , officials told CNN . A border guard was killed in a cross-boundary fire exchange with militants , the Saudi Press Agency reported Thursday . That marked Saudi Arabia 's first publicly known military death since it launched airstrikes against rebels inside its southern neighbor . U.S. Navy warships were patrolling off Yemen in search of suspicious shipping in the region , in particular any weapons shipments from Iran to the Houthi rebels they are supporting in Yemen , a U.S. defense official told CNN on Thursday . The Navy this week came across a Panamanian-registered cargo ship that had left an Iranian port , the official said . Based on the route of the ship from Iran to Yemen , the Navy declared it a `` vessel of interest '' and requested permission from the captain to search it . The ship was found to be carrying cement and construction materials , the official said . It was sent on its way . The offshore area has been a route for weapons smuggling into Yemen , as well as Sudan and Gaza , so the Navy has maintained a regular surveillance presence there . The official said that increased fighting in the last several days in southern Yemen has resulted in other ships trying to dock to help civilians leave the country . Saudi and allied warplanes struck rebels in Yemen last week , with Saudi Arabia threatening to send ground troops and inserting itself into its southern neighbor 's civil war . The swift and sudden action involved 100 Saudi jets , 30 from the United Arab Emirates , 15 each from Kuwait and Bahrain , 10 from Qatar , and a handful from Jordan , Morocco and Sudan , plus naval help from Pakistan and Egypt , according to a Saudi adviser . The Houthi rebels are Shiite Muslims who have taken over Sanaa and had captured parts of its second-largest city , Aden . The Sunni Saudis consider the Houthis proxies for the Shiite government of Iran and fear another Shiite-dominated state in the region . Officials : Al Qaeda fighters free 270 from Yemeni prisons . CNN 's Barbara Starr contributed to this report .
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 .