Terrorism crosses into Saudi Arabia from Yemen, a country where Al-Qaeda finds a sanctuary under deals with tribal leaders, a Saudi prince has said.

Yemen is a direct security threat to the Saudi kingdom, and its instability because of the rebels in the far north and the secessionist movement in the south is helping it falling apart, and then enabling Al-Qaeda to be stronger and act freely, Turki Al-Faisal, an ex-ambassador to the United States, said on Friday.

"The situation in Yemen in general is making it easy for terrorists to infiltrate into Saudi Arabia and operate actively there. Also, many Yemenis flee into the kingdom amid conflicts and deteriorating humanitarian situation in their country."

At a lecture at the Harvard Kennedy School, the former Saudi intelligence service chief said the current situation in Yemen represents a serious security threat to its neighbor along the border and internally.

With the situation worsening, the security threat would be even more serious, he warned.

The Yemeni tribes are protecting Al-Qaeda as do Pakistani tribes, with terrorists making deals with tribal elders for supplies and protection, he added.

Saudi Arabia should deploy national forces along the border with Yemen and it should do what she can to safeguard its national security, he urged.