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

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 the kingdom would have launched the offensive without tacit Yemeni agreement.

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 underway 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 sensitivity of the issue. He said Saudi troops were coordinating with Yemen's army, but Yemen's defense ministry denied 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 that is increasingly allied with hard-line Sunni fundamentalists, who consider Shiites heretics.

The rebels said the Saudi airstrikes hit five areas in their northern stronghold Thursday but it was not possible to independently verify the reports. They said there were dead and wounded, and that homes were destroyed.

"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."