Yemen’s Houthis on Thursday launched a missile at a camp housing Yemeni forces belonging to a Saudi-led coalition that had sent reinforcements to counter a push on the oil-producing province of Shabwa, military sources and a local official said.

The ballistic missile killed four soldiers and wounded 13 at the camp in Markha, three military sources said. A local official put the death toll at seven.

There was no immediate comment from the Houthis or from the pro-coalition Giants Brigade, which said on Wednesday that fighters previously stationed in western Yemen had arrived in Shabwa in the south.

The Iran-aligned Houthis, who have been battling the coalition for more than six years, has advanced in the central province of Marib, the country’s only gas-producing area and home to its largest oil fields.

The group has made inroads into Shabwa, cutting off supply lines to Marib, which is the internationally recognised government’s last northern stronghold. Government forces still hold Marib’s capital and nearby hydrocarbon facilities.

The war, which has killed tens of thousands and displaced millions, has been in a military stalemate for years but 2021 saw shifts in frontlines that could further complicate stalled peace efforts.

The Houthis have also stepped up cross-border missile and drone attacks on Saudi Arabia and the coalition has carried out air strikes on Houthi targets, including in the capital, Sanaa.

U.N. special envoy Hands Grundberg warned on Dec. 14 that the escalation could open an “even more fragmented and bloody” chapter in the war which has caused a dire humanitarian crisis.

Yemen has been mired in violence since the Houthis ousted the government from Sanaa, prompting the coalition to intervene months later in March 2015 in a conflict largely seen as a proxy war between Saudi Arabia and Iran.

The Houthis say they are fighting a corrupt system and foreign aggression.