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The UN secretary-general has said that two weeks of Saudi-led air strikes against Houthi rebels in Yemen, and the rebels' advances "have turned an internal political crisis into a violent conflict that risks deep and long-lasting regional repercussions".

Ban Ki-moon on Thursday told reporters that he was urging all countries in the region to go beyond national priorities and help the Yemeni people, saying "the last thing the region and our world need is more of the chaos and crimes we have seen in Libya and Syria".

He has called on all parties to the conflict to allow the delivery of badly needed aid, after the World Health Organisation said on Wednesday that at least 643 civilians and combatants have been killed, and 2,200 injured.

Tens of thousands of families have also been displaced and the WHO has warned that the situation in the Arabian Peninsula nation is critical.

Ban says UN-led talks are the best solution.

Yemenis fleeing violence detained in Saudi Arabia

Houthis capture Ataq

Ban's comments came after Houthi fighters, backed by supporters of former Yemeni President Ali Abdullah Saleh, entered the provincial capital of the Shabwa province in eastern Yemen, despite the intense Saudi-led air strikes against the group.

Residents said local tribal chiefs and security officials facilitated the entry of the Houthi forces to the city of Ataq on Thursday, where they took control of the offices of the local government and security forces compounds.

It was the first time that the Houthis and their allies had entered the city, bringing them closer to the country's most prized economic asset, the Belhaf natural gas facility and export terminal, on the Arabian Sea about 160 km to the southeast.

A great nation like Yemen will not submit to bombing. Come, let us all think about ending war. Let us think about a ceasefire...Let us prepare to bring Yemenis to the negotiating table.

Hassan Rouhani, Iranian president

Saudi Arabia, backed by four Gulf Arab states and other regional Arab allies, has mounted two weeks of air strikes against the Houthis, who have since pushed south towards Aden, the stronghold of Saudi-backed Yemeni President Abd Rabbu Mansour Hadi.

The air campaign failed to stop the Houthis and soldiers loyal to Saleh entering central Aden, but the coalition says it has cut Houthi supply lines, destroyed weapons depots and pushed them back in some southern provinces around Aden.

In a speech on Thursday in Tehran, Iran's President Hassan Rouhani called for an end to the air strikes in Yemen, saying countries in the region should work towards a political solution.

"A great nation like Yemen will not submit to bombing. Come, let us all think about ending war. Let us think about a ceasefire...Let us prepare to bring Yemenis to the negotiating table," he said.

Saudi Arabia and its Gulf allies accuse Iran of arming the Houthi fighters but Iran denies the allegations.

Al-Qaeda continues its advance

Earlier in the day, residents of al-Siddah district in central Yemen said they woke to find al-Qaeda flags flying over local government offices, Reuters news agency reported.

They said a group of al-Qaeda gunmen led by a local commander known as Ma'mour al-Hakem, took over the district at night. Residents said the Houthis, who had been in control of the town for more than two months, retreated without a fight.

The United States defence secretary has said that the escalated conflict has provided al-Qaeda with an "opportunity" to gain new territory in Yemen. This is despite confirmation from the US that it is expediting advanced arms shipments to Saudi Arabia.

Yemen infographic [Al Jazeera]

Source: Al Jazeera and agencies