ADEN (REUTERS) - Houthi militia forces in Yemen backed by allied army units seized a key air base on Wednesday and appeared poised to capture the southern port of Aden from defenders loyal to President Abd-Rabbu Mansour Hadi, local residents said.

After taking al-Anad air base, the Houthis and their military allies, supported by heavy armour, advanced to within 40km of Aden, where Mr Hadi has been holed up since fleeing the Houthi-controlled capital Sanaa last month.

Unidentified warplanes fired missiles at the Aden neighbourhood where Mr Hadi's compound is located, residents said. Anti-aircraft batteries opened fire on the planes.

Yemen's slide towards civil war has made the country a crucial front in mostly Sunni Saudi Arabia's rivalry with Shi'ite Iran, which Riyadh accuses of stirring up sectarian strife through its support for the Houthis.

Sunni Arab monarchies around Yemen have condemned the Shi'ite Houthi takeover as a coup and have mooted a military intervention in favour of Mr Hadi in recent days.

U.S. officials say Saudi Arabia is moving heavy military equipment including artillery to areas near its border with Yemen, raising the risk that the Middle East's top oil power will be drawn into the worsening Yemeni conflict.

While the battle is publicly being waged by the Houthi movement, many in Aden believe that the real instigator of the campaign is former president Ali Abdullah Saleh, a fierce critic of Mr Hadi.

It was Mr Saleh who was the author of Aden's previous humiliation in 1994, when as president he crushed a southern secessionist uprising in a short but brutal war.

Unlike other regional leaders deposed in the Arab Spring, Mr Saleh was allowed to remain in the country.

Army loyalists close to Mr Saleh on Wednesday warned against foreign interference, saying on his party website that Yemen would confront such a move "with all its strength".

Diplomats say they suspect the Houthis want to take Aden before an Arab summit this weekend, to preempt an expected attempt by Mr Hadi ally Saudi Arabia to rally Arab support at the gathering for military intervention in Yemen.

Yemeni officials denied reports that Hadi had fled Aden.

"The war is imminent and there is no escape from it," said 21-year-old Mohammed Ahmed, standing outside a security compound in Aden's Khor Maksar district, where hundreds of young men have been signing up to fight the advancing Shi'ite fighters. "And we are ready for it".