Yemen is facing a food crisis of "catastrophic proportions", with almost half the population going hungry and a third of children in some areas severely malnourished, aid agencies have warned.

A coalition of seven humanitarian organisations – Care International, International Medical Corps, Islamic Relief, Mercy Corps, Merlin, Oxfam and Save the Children – is urging the international community to step up aid before Yemen slides further into poverty and political instability.

The Middle Eastern country's already precarious state was highlighted on Monday when a suicide bomber attacked a military parade, killing more than 90 people and wounding at least 220. The bombing, one of the deadliest in recent years, was a setback in Yemen's battle against al-Qaida-affiliated Islamists and has heightened concerns over a country in the frontline of the US global war on militants.

The aid agencies – which point out that the UN's humanitarian appeal has received only 43% of the funding it needs – are urging delegates at Wednesday's international Friends of Yemen conference to do more to tackle the food crisis.

The agencies say hunger in Yemen has doubled since 2009 and was exacerbated by last year's political upheaval, which saw the dictator Ali Abdullah Saleh ousted from power after 33 years and replaced by President Abd-Rabbu Mansour Hadi.

Penny Lawrence, Oxfam's international director, said that although donors were concentrating on politics and security, they had to focus on more basic and pressing issues. "Yemeni families are at the brink and have exhausted their ways of coping with the crisis," she said.

"Failure to respond adequately to the humanitarian needs now will put more lives at risk, further entrench poverty and could undermine political transition in the country."

Her fears were echoed by Jerry Farrell, Save the Children's director in Yemen. "Political instability, conflict and high prices have left families across the country going hungry," he said. "We know that children always suffer the most when food is in short supply, and unless urgent humanitarian action is taken, Yemen will be plunged into a hunger crisis of catastrophic proportions."

Fighting in the north and south of the country has forced nearly 95,000 people to leave their homes over the last two months, bringing the total number of those displaced in the country to close to half a million.

UN estimates, meanwhile, put the number of children facing life-threatening levels of malnutrition at 267,000. Oxfam has also reported an increase in early marriage as families marry off their daughters young in order to ease the burden of the crisis.

Although the UN has so far asked for $447m [£283m] of aid, it is thought it will increase its appeal substantially next month as the situation in Yemen worsens.

"The hungry of Yemen cannot wait," said Hashem Awnallah, country director of Islamic Relief Yemen. "Donors need to heed the lessons of the Horn of Africa and respond now before the crisis further deepens."

Britain, which will co-chair the Friends of Yemen conference in Riyadh, has promised £28m of aid to the country, but warned that Monday's suicide bombing underlined the severity of the situation in Yemen.

The international development minister, Alan Duncan, said the aid – which will be delivered through agencies – would provide both immediate help and a "foundation for progress". But Duncan, who will be among those at the meeting, said that Yemen was still in a very fragile state. "The new government of Yemen has been in place for a matter of months and has already taken important steps," he said. "But yesterday's terrible suicide bombing reminds us that the country still faces huge challenges.

"If progress is to be maintained then the international community must back the government. Without that support, the alternative is a slide towards state failure and an increased threat from international terrorism."

Dfid officials said the aid could mean emergency food for up to a quarter of a million people as well as safe water sources, shelter and healthcare for those fleeing the violence.

It could also help fund education for 60,000 children in conflict zones and pay for goods and labour to support 35,000 adults.

Speaking at the Nato summit in Chicago on Monday, Barack Obama also expressed concern over extremist activity in Yemen and pledged more aid to counter it.

"That's important for US safety," he said. "It's also important for the stability of Yemen and the region."

Obama's counter-terrorism adviser, John Brennan, telephoned Hadi to offer US help in the investigation into Monday's bombing, adding that it "would stand by Yemen's side at this difficult time".

Washington is increasing its support for Hadi's government but the US military's drone attacks targeting militants in Yemen have frequently killed civilians and are deeply resented by Yemenis, even the many who abhor al-Qaida.