AL MUKALLA, Yemen — Houthi rebels in Yemen hit a Saudi Arabian oil tanker with a missile on Tuesday, an attack that could escalate the three-year-old conflict even as the leader of the United Nations expressed optimism about renewed diplomacy to end it.

In a statement carried on Al Mayadeen television in Yemen, the Houthis said the missile strike avenged the Yemeni civilians killed Monday in a Saudi aerial assault on Al Hudaydah, the only Yemeni seaport controlled by the rebels. At least 14 people, including children, were killed after they ventured outside their residential building to get some fresh air in the stifling midmorning heat, witnesses said.

The Saudi-led military coalition, which has been bombing the Houthis and their allies since March 2015, has said its attack on Al Hudaydah struck military targets.

The Houthis have no air force. But they have enraged the Saudis by repeatedly firing missiles into Saudi Arabia, most notably a seven-missile barrage launched late last month. The Saudis claim that the weapons are supplied by Iran, Saudi Arabia’s regional rival. Iran denies it.