SAN'A, Yemen -- Saudi Arabia sent fighter jets and artillery bombardments across the border into northern Yemen yesterday in a military incursion apparently aimed at helping its troubled southern neighbor control an escalating Shiite rebellion, Arab diplomats and the rebels said.

The Saudis -- owners of a sophisticated air force they rarely use -- have been increasingly worried that extremism and instability in Yemen could spill over to their country, the world's largest oil exporter. The offensive came two days after the killing of a Saudi soldier, blamed on the rebels.

Yemen denied any military action by Saudi Arabia inside its borders. But Yemen's president is a key ally of the Saudis, making it highly unlikely that the kingdom would have launched the offensive without tacit Yemeni agreement.

A U.S. government official said the Yemenis were not involved militarily in the fighting. The official spoke anonymously because he was not authorized to discuss the matter publicly.

The offensive immediately raised concerns of another proxy war in the Middle East between Iran and Saudi Arabia, a key U.S. ally. Shiite Iran is believed to favor the rebels in Yemen, while Saudi Arabia, which is Sunni, is Iran's fiercest regional rival.

The same dynamic has played out in various forms in Lebanon, where Iran supports the Shiite militant Hezbollah and Saudi Arabia favors a U.S.-backed faction, and in Iraq, where Saudi Arabia and Iran have thrown support to conflicting sides in the Sunni-Shiite struggle.

A top Saudi government adviser confirmed "a large-scale" military operation under way on the Saudi-Yemeni border with further reinforcements sent to the rugged, mountainous area. "It is a sustained operation which aims to finish this problem on our border," he said, speaking on condition of anonymity because of the issue's sensitivity. He said Saudi troops were coordinating with Yemen's army, but Yemen's defense ministry denied that the Saudis were inside the country.

The northern rebels, known as Hawthis, have been battling Yemeni government forces the past few months in the latest flare-up of a sporadic five-year conflict. They claim their needs are ignored by a Yemeni government increasingly allied with hard-line Sunni fundamentalists, who consider Shiites heretics.

The rebels said the Saudi airstrikes hit five areas in their northern stronghold yesterday, but it was not possible to verify the reports independently. They said there were dead and wounded, and that homes were destroyed. The rebels' spokesman said people were afraid to get near the areas being bombed, making it difficult to count the casualties.

"Saudi jets dropped bombs on a crowded areas, including a local market in the northern province of Saada," Hawthi spokesman Mohammed Abdel-Salam told The Associated Press. "They claim they are targeting al-Hawthis, but regrettably, they are killing civilians like the government does."

He said the attacks were followed by hundreds of artillery shells from the border. "So far, three killed have been pulled out of the rubble, including a woman and a child who perished when their houses were bombed and burned down," he said.

The fighting is more than 600 miles from Saudi Arabia's oil fields on the kingdom's eastern Persian Gulf coast. But northern Yemen overlooks the Red Sea, the world's busiest oil tanker route.

Two Arab diplomats, speaking on condition of anonymity, said Saudi Tornado and F-15 warplanes had been bombarding targets inside Yemen since Wednesday afternoon, inflicting significant casualties on rebels. They said army units and special forces also had been sent to northern Yemen, and that several Saudi towns on the border had been evacuated as a precaution. The diplomats spoke on condition of anonymity because they are not allowed to talk to the media.

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