Fighting between Yemen’s warring parties in the gas-rich Marib region, the recognized government’s last northern stronghold, intensified late on Saturday, three sources said, at a time the United Nations and United States are pushing for a peace deal.

The Iran-aligned Houthis, who have been battling a military coalition led by Saudi Arabia for over six years, have been trying to seize Marib in an offensive described by Washington as the most serious threat to efforts to achieve a truce.

Battles had abated as diplomatic efforts ramped up in recent weeks, but three pro-government Yemeni sources said tens of fighters from both sides were killed in fighting after a fresh Houthi assault that was met with intense coalition air strikes.

Houthi-run Al Masirah television channel said coalition warplanes conducted 13 strikes late on Saturday.

“The fighting continued until the early morning,” one of the sources, a local official, said. “They are the heaviest in weeks.”

Marib, which hosts some 1 million internally displaced people, has become the focal point of a war that has killed tens of thousands of Yemenis and pushed the Arabian Peninsula nation to the brink of famine.

The conflict, widely seen in the region as a proxy war between Saudi Arabia and Iran, has been in military stalemate for years with the Houthis holding most big urban centers.

The warring parties have been thrashing out terms for a U.N.-led proposal to remove restrictions on Houthi-held ports and Sanaa airport to alleviate a dire humanitarian crisis and for a ceasefire that is needed to revive political negotiations last held in late 2018.

The Houthis, whose hand would be strengthened in any future talks by taking Marib, have insisted the blockade be lifted before any truce talks. The coalition wants a simultaneous deal.

The military alliance intervened in Yemen in March 2015 after the Houthis ousted the Saudi-backed government from the capital Sanaa.

The Houthis, who have repeatedly launched cross-border missile and drone attacks on Saudi cities, largely intercepted, say they are fighting a corrupt system and foreign aggression.