An advisor to the Riyadh government says the Saudi army has created a military buffer zone deep inside the embattled Yemeni North, using heavy air and artillery fire.

The area has been described as a no-go killing zone by Saudi media.

Last week, the Saudi military formally launched an offensive against Houthi fighters who were taking refuge near the border between the two countries.

Sana'a started a massive crackdown in August after irregular battles against the Houthis since 2004, when the group took arms against the Sunni-dominated central government in protest to its repression and discrimination against the Shia minority of the county.

Some 150,000 Yemenis have been forced out of their homes in the beleaguered north with the ongoing Operation Scorched Earth, also joined by Saudi military forces and fighter jets, accounting for an estimated 55,000 of the displacements.

The anti-government fighters accuse Saudi Arabia of collaborating with the Yemeni government in its military offensive on the Shia forces and of targeting civilians in villages far from its territory.

They also charge Sana'a with employing al-Qaeda mercenaries and terrorists to help the army with its campaign against Houthis.

The United States has recently joined the conflict by signing a military agreement with Yemen under which Washington will provide Sana'a forces with intelligence and training to quell Houthi movement.

The fighters' leader, Yahya al-Houthi, on Thursday called on Riyadh to agree to a ceasefire and expressed his group's willingness to negotiate with Saudi security officials.

The Kingdom has threatened to continue its attacks on the Shia fighters until they move back into Yemen from the 'frontier' between the two countries.

The impoverished province of Sa'ada and neighboring Amran, flash points of conflict in northern Yemen, remain cut off from the rest of the country and are frequently bombarded by military fighter jets.

MRS/HGH