The conflict and chaos in Yemen has opened the door to al Qaeda, and there needs to be international mediation to end the violence rather than unilateral action, the European Union's foreign policy chief said on Wednesday.

A Saudi-led Arab alliance has been conducting air strikes against Houthi fighters in Yemen since late March.

Iranian-allied Houthi fighters, backed by forces loyal to former President Ali Abdullah Saleh, have seized control of large parts of Yemen in what they say is a campaign against al Qaeda militants in the country.

The Saudi-led coalition, including nine Arab states provided with logistical support by the United States, France and Britain, seeks to restore the government of President Abd-Rabbu Mansour Hadi, now in exile in Riyadh.

Speaking to students during a visit to Beijing, EU foreign policy boss Federica Mogherini said the situation in Yemen was highly hazardous.

"I believe that this is extremely dangerous, as al Qaeda is trying to fill the gap, the vacuum, that is left open in a state of increasing lawlessness," Mogherini said at the elite Peking University.

"We need to focus all efforts we can on an international mediation," she added.

"Because we know that unilateral solutions don't work in general, and in particular don't work in a situation like the one in Yemen where the conflict has deep local roots and in the meantime is inserted into a complex regional situation."

The EU was grateful to China for helping evacuate foreign residents from Yemen, Mogherini said, calling for China to get even more involved in crises such at this one, especially as what happened in Yemen could impact upon China too.

"Not only Europe but also China could be affected by these developments, in terms of security," she added.

China, a low-key diplomatic player in the Middle East despite its reliance on oil from the region, has called for a ceasefire in Yemen and a political solution.