(Bloomberg) -- Saudi-led coalition warplanes struck military positions belonging to Iran-backed rebels in Yemen’s capital, hours after the kingdom proposed a negotiated end to a war that’s raged for six years and created the world’s worst humanitarian crisis.

Rebel-run Al-Masirah television said planes targeted a military camp and an air base in Sana’a, as well as other areas overnight. Residents reported hearing explosions and fighter jets hovering over the city. There was no immediate confirmation or comment from Saudi Arabia.

On Monday, Saudi Arabia proposed a series of steps including a comprehensive cease-fire to be supervised by the United Nations, to begin as soon as the Houthi rebels accepted the plan. The group appeared to reject the proposal almost immediately, saying it would not countenance talks until Saudi Arabia brings its military campaign to a complete halt and lifts a blockade on the airport and ports. Saudi Arabia says there is no blockade.

Iran, which has helped supply and train the Houthis, said Tuesday that a Yemen cease-fire would help to pave the way for dialogue if implemented in tandem with an end to the blockade.

The Houthis have been fighting Yemen’s internationally recognized government since 2014, capturing Sana’a and swaths of the country. A Saudi-led coalition intervened the following year on the side of the government.

U.S. President Joe Biden has vowed to end the conflict, ceasing sales of offensive weapons to Saudi Arabia, ending the Houthis’ terrorist designation to facilitate the delivery of humanitarian aid, and naming a special envoy to push for a negotiated settlement. The U.S. push has given momentum to a UN peace effort already in the works. Both the UN and U.S. welcomed the Saudi initiative.

Fighting has intensified as the diplomatic efforts have gained momentum, with the rebels pushing to capture the strategic city of Marib, an energy-producing area and government stronghold. In recent weeks, battles have spread to other areas, including Taiz and Hajjah, where government forces are seeking to make gains.

There’s also been an uptick in Houthi-claimed attacks deep inside Saudi Arabia itself, mainly targetting oil and military infrastructure. They come as Biden clashes with Iran over how to revive a 2015 accord designed to curb its nuclear program, leading to claims that Iran is using the group to pressure the U.S. to remove sanctions.

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