SAN'A, Yemen — Saudi warplanes and artillery bombarded a Shiite rebel stronghold in northern Yemen Saturday 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."

"The war just started two days ago, and what has been happening in the past six years was just ... training for the army units," he said. "The war will never stop no matter how much money or martyrs it costs."

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.

It was not possible to independently verify rebel reports that the Saudi shelling and airstrikes continued Saturday. After nightfall, a correspondent for Saudi-owned Al-Arabiya TV reported from the border area that he could hear heavy bombardments. There was no information on casualties.

Saudi forces raided a home, capturing three Yemeni fighters hiding there and seizing radio communications equipment, Al-Arabiya reported. Troops were also forcibly evacuating residents of border areas, it said.

Besides the northern rebels, Yemen's government is also confronting a separatist movement in the south and a lingering threat from al-Qaida militants.

Yemen, the ancestral homeland of Osama bin Laden, has cooperated with the U.S. in fighting terrorism but has struggled to confront Islamic extremists.