The United Nations is warning that an offensive by Houthi rebels in Yemen has escalated the nearly six-year conflict in the Arab world’s poorest nation as it “speeds towards a massive famine.”.

The United Nations warned Tuesday that an offensive by Houthi rebels in Yemen has escalated the nearly six-year conflict in the Arab world’s poorest nation as it “speeds towards a massive famine.”

U.N. special envoy Martin Griffiths and U.N. humanitarian chief Mark Lowcock painted a grim picture of the political, economic and humanitarian crisis in Yemen, which is exacerbated by a government blockade of fuel ships entering the country’s main port of Hodeida controlled by the Houthis.

The intensified fighting has come amid an international and regional diplomatic push to end the conflict which began with the 2014 takeover of the capital Sanaa by the Iranian-backed Houthis. A Saudi-led coalition supported by the U.S. and allied with the government has been fighting the rebels since March 2015.

U.S. President Joe Biden’s envoy to Yemen, Tim Lenderking, last week urged the Houthis to agree to a cease-fire proposal.

Griffiths told the U.N. Security Council the Houthis’ weeks-long offensive on the oil-rich central province of Marib, the government’s last stronghold in Yemen’s northern half, has put an estimated one million already displaced civilians at risk. Fighting forces on both sides have suffered “heavy losses,” he said.

Missile and drone strikes from Houthi-controlled areas into Saudi Arabia targeting civilian and commercial infrastructure “have also increased significantly in recent weeks,” Griffiths said. And retaliatory airstrikes on Sanaa city are “endangering civilians there as well.”

Griffiths said other fronts have also opened, with government forces earlier this month launching an offensive against rebel positions in western Hajjah province, and fighting in the government-held southern province of Taiz.

The result has been “a dramatic deterioration” in the conflict, he said.

In addition to the suffering of Yemenis, Griffiths called for an independent investigation into the cause of last week's fire at a Houthi detention center in Sanaa holding predominantly Ethiopian migrants that killed dozens and seriously injured over 170.

On the humanitarian front, Yemen, which imports most of its food and other commodities, remains the world’s worst crisis.

Both Griffiths and Lowcock urged the entry of fuel ships to Hodeida, warning that the blockade since January has increased prices of food and other goods and put hospitals and other services, including water which needs fuel for its pumps, at risk.

“Right now, 13 fuel ships are waiting outside Hodeida, carrying two months of imports,” Lowcock said, explaining that the government isn’t clearing them because of a dispute with the Houthis over revenue from the fees and taxes on the oil ships.

Despite the recent escalation in fighting in Marib and elsewhere, Lowcock told the Security Council that “the renewed U.S. commitment to a diplomatic solution has opened a window for anyone who is serious about ending the war."

He again warned that “Yemen is speeding towards a massive famine" and said “that opportunity will be wasted if Yemen tips into famine.”

“So, I again call on everyone to do everything you can -- including money for the aid operation -- to stop the famine,” the undersecretary-general for humanitarian affairs said.

A U.N. pledging conference on March 1 raised a disappointing $1.7 billion, less than half of what aid agencies need this year, which means “we don’t have enough money to stop famine,” Lowcock said.

Last week, U.N. World Food Program chief David Beasley, who had just returned from Yemen, told the council it was “hell on earth in many places” and warned that “we are heading straight toward the biggest famine in modern history.”

“Over 16 million people now face crisis levels of hunger or worse,” Beasley said.

Griffiths said with famine now arriving, “a nationwide cease-fire, along with opening Sanaa airport and ensuring the unhindered flow of fuel and other commodities into Yemen through Hodeida ports, are urgent humanitarian imperatives.”

The U.N. envoy said the U.S. engagement “gives us all more energy and a great deal of hope.”

But Griffiths expressed alarm that “the mere fact of meeting across the table to discuss the contours of ending the war is being framed as a concession rather than an obligation.”