SANAA, Yemen -- Saudi-led warplanes bombed Shiite rebels in their northern Yemeni stronghold on Thursday as fighting raged in an oil-rich eastern province, security officials and tribesmen said.

As the Saudi-led campaign against the Iran-backed rebels, known as Houthis, entered its third month, Amnesty International warned that civilians were trapped in the crossfire. The Houthis and allied military units loyal to a former president are battling forces loyal to internationally-recognized President Abed Rabbo Mansour Hadi, who fled to Saudi Arabia in March.

Warplanes fired at least 20 missiles at Houthi positions in their northern heartland of Saada and bombed Aden University in the southern port city, officials said, adding that the university is believed to be used as a Houthi hideout.

Fierce clashes between pro-government tribal fighters and Houthis were meanwhile underway in the eastern province of Marib, home to much of Yemen's oil industry. Tribesmen said 10 civilians have been killed the past 48 hours.

The officials and the tribesmen spoke on condition of anonymity because they were not authorized to brief reporters.

International aid groups say Yemen's conflict has killed up to 2,000 people and wounded 8,000, while recent U.N. estimates have said that at least 1,037 civilians, including 130 women and 234 children, have been killed in the fighting.

London-based Amnesty International said in its report Thursday that residents in the rebel-held capital, Sanaa, are "caught in a deadly crossfire between the Saudi Arabian-led coalition airstrikes and anti-aircraft fire from the Houthi armed group."

Amnesty's Senior Crisis Adviser Lama Fakih said "both sides have displayed a chilling indifference to the deadly impact of their actions on civilians. All parties to the conflict can and should take all feasible steps to minimize the risk to civilians."