RIYADH (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 has 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 Sanaa, 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 (March 22), Saudi Arabia proposed a series of steps including a comprehensive ceasefire 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 on Tuesday that a Yemen ceasefire 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 recognised government since 2014, capturing Sanaa and swaths of the country.

A Saudi-led coalition intervened the following year on the side of the government.

US 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 US push has given momentum to a UN peace effort already in the works. Both the UN and US welcomed the Saudi initiative.

Fighting has intensified as the diplomatic efforts 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 has also been an uptick in Houthi-claimed attacks deep inside Saudi Arabia itself, mainly targeting oil and military infrastructure.

They come as Mr Biden clashes with Iran over how to revive a 2015 accord designed to curb its nuclear programme, leading to claims that Iran is using the group to pressure the US to remove sanctions.