UPDATE 1-Anti-aircraft guns fire at plane over Aden-witnesses

ADEN, March 22 (Reuters) - Yemeni anti-aircraft guns opened fire at an unidentified plane flying over President Abd-Rabbu Mansour Hadi's compound in the southern city of Aden on Sunday and appeared to force it away, witnesses said.
It was the third incident of its kind in the past four days, in which unidentified aircraft have flown over the compound, where Hadi is based, on one occasion dropping bombs without causing any casualties.
Aden's governor Abdulaziz bin Habtoor has accused the Shi'ite Muslim Houthi movement of ordering the flights, an allegation the Iran-allied group, which controls much of the north of the country, has yet to address.
Yemen has descended into civil war since last year when the Houthis seized Sanaa and removed Hadi from effective control of the state. The Houthis then advanced into Sunni Muslim areas, leading to clashes with local tribes and al Qaeda.
On Saturday Hadi, who fled to Aden from Sanaa last month, accused the Houthis of staging a coup against him and appealed to the United Nations for "urgent intervention".
Eyewitnesses in the central province of Ibb described to Reuters seeing a column of dozens of tanks and military vehicles travelling from the Houthi-loyalist north on their way southward toward Taiz, 150 km (200 miles) northwest of Aden.
The Houthis are allied with former Yemeni president Ali Abdullah Saleh, who still wields influence in the armed forces despite having given up power in 2011 after mass protests against his rule.

(Reporting by Mohammed Mukhashaf, writing by Sami Aboudi, Editing by William Maclean and Louise Heavens)