While millions of Yemenis at risk of starvation waiting for food aid, grain silos outside the port of war-torn Hodeida were hit by gunfire just days after aid staff gained access.

The World Food Program (WFP) has assessed the damage from the Thursday attack, which caused no casualties, the U.N. agency's senior spokesperson, Herve Verhoosel, said in a statement. Verhoosel said the silos contained some 51,000 tons of grains, enough to feed 3.7 million people for a month. "It appears that two silos were hit, one of which contains wheat," he said. "There was no fire." Verhoosel told said the WFP was unaware who carried out the attack.

In the midst of an escalating humanitarian crisis in the war-stricken country, the U.N. regained access to donated grain stored in the Yemeni port city of Hodeida last Sunday and began the task of salvaging food that could stave off starvation for millions of citizens before it rots. WFP grain stores there have been cut off for eight months, putting 51,000 tons of wheat at risk of rotting. The stores came under the control of government forces after fierce battles last year but a major front line is only a few blocks away.

The U.N. expects that process to take several weeks before starting to mill it into flour and distributing it to the Yemeni communities most in need. An assessment carried out in February, when the U.N. was briefly granted access to the mills for the first time since September, concluded that around 70 percent of the wheat might be salvageable. But the flour yield will be lower than normal as weevil infestation has caused hollow grains, the U.N. said, based on that assessment. Talks aimed at securing a mutual military withdrawal from Hodeida have stalled despite U.N. efforts.

Hodeida is the main port of the impoverished Arab country – where around 8.4 million people are believed to be on the verge of starvation – and a lifeline for millions.

The fighting has largely stopped in the city under a U.N.-brokered ceasefire struck in Sweden in December but there have been intermittent clashes.

Saudi Crown Prince Mohammed bin Salman, the former Saudi defense minister and Saudi Arabia's allies launched Operation Decisive Storm in March 2015. The United Arab Emirates (UAE) has largely handled ground operations in the conflict, while the Saudis have bombed the country from the air. Many atrocities have been reported so far, which reveals multiple violations of human rights. Saudi airstrikes have hit markets and hospitals, killing civilians. The coalition led by Saudi Arabia and the UAE has been using weapons produced in Europe and the U.S. to kill and wound hundreds of civilians in Yemen.

The more than four-year conflict in Yemen has killed tens of thousands of people, many of them civilians, relief agencies say.

The fighting has triggered what the United Nations describes as the world's worst humanitarian crisis, with 3.3 million people still displaced and 24.1 million -- more than two-thirds of the population -- in need of aid.