Al-Jazeera is reporting that Saudi ground forces have entered Yemen in what seems to be a large-scale offensive.

A statement from the Houthi rebels said: "The Saudis began an attack along many fronts on the Yemeni border ... The Saudi army has been using all kind of weapons; land, air, tanks, and artillery."

Despite an official Yemeni denial (which on past form can't be taken very seriously), witnesses in the northern border area of Yemen confirmed the attack to AFP. "The Saudi army launched a vast offensive against Huthi positions," one is quoted as saying.

If the reports are correct, this is a very serious development – and probably a major blunder on the Saudis' part. As Mai Yamani notes in an article today:

The Saudis are unlikely to succeed militarily in Yemen. Yemen's army of 700,000 could not suppress the Houthi rebellion, despite five attempts since 2004. Now they are leaving Saudi Arabia's untested army of 200,000 men to do the job for them. And, while the Saudis are currently relying on their air force, a full-scale land battle will have to follow – on the same harsh terrain that helped defeated Nasser's battle-hardened troops in the 1960s.

Saudi military involvement on Yemeni soil will also be unpopular in Yemen (not just among Houthi supporters) and could prove extremely damaging to President Salih politically.

Posted by Brian Whitaker, 23 November 2009, 20.00 GMT.

UPDATE 25 November 2009: Saudi Arabia officially denied that its forces entered Yemeni territory.