Saudi warplanes and artillery bombarded a Shiite rebel stronghold in northern Yemen yesterday for a third straight day, according to the rebel fighters, and Yemen's president vowed to wipe out the insurrection.

The sporadic five-year conflict between Yemen's weak central government and rebels in the north of the impoverished country escalated dramatically this week when Saudi military forces began shelling and bombing rebel positions.

The two nations are cooperating and sharing intelligence in the fight. Officials in neighboring Saudi Arabia have been increasingly worried that extremism and instability in Yemen could spill over to their country, the world's largest oil exporter.

Publicly, Saudi officials say their military action has been limited to areas inside its own borders, targeting fighters who have crossed into its territory. But Yemeni rebels, military officials and Arab diplomats say the Saudi air and artillery strikes began to hit deep inside northern Yemen on Thursday.

Yemeni President Ali Abdullah Saleh said there will be no let up in the battle "until we bring this tyrannical, traitorous and mercenary group to an end."

Mr. Saleh was speaking to hundreds of government officials and foreign investors gathered in southern Yemen for the inauguration of the nation's first gas plant.

The northern rebels, known as Hawthis, claim the needs of their communities are ignored by a Yemeni government that is increasingly allied with hard-line Sunni fundamentalists, who consider Shiites heretics.

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