As a unilateral 5-day humanitarian pause declared by the Saudi-led coalition bombing Yemen appeared to crumble, the aid group Oxfam said Tuesday that half of Yemen's population — almost 13 million people — is struggling to obtain food, and that some 6.5 million people are "on the brink of starvation."

Oxfam said that since the start of conflict between the coalition and Houthi rebels in March, an average of 25,000 additional Yemenis went hungry every day. The worst hit area is Saada governorate, a Houthi stronghold which has been bombarded relentlessly by coalition jets. Fifty percent of the people in Saada face "critical" levels of hunger, the group reported.

Even prior to the outbreak of violence, Yemen was the poorest country in the Arab world, and imported the vast majority of its food, predominantly by sea. Last year, the UN's World Food Program estimated that 10.6 million Yemenis were already food insecure.

Since airstrikes began on March 26, the Saudi-led coalition has installed a de-facto blockade of the country, leading to sparse supplies of basic necessities such as fuel, cooking gas and food like wheat and rice. While the UN and partner organizations are running desperately low on funds in Yemen, Oxfam said even with proper financing, the obstruction of commercial routes by the coalition would prevent locals from accessing the food and fuel they need to survive.

"These numbers are extremely alarming," Tariq Riebl, head of programs for Oxfam in Yemen, told VICE News. "People may just make a direct link to the conflict, but the actual reason for a lot of the food insecurity is the economic blockade."

"They are primarily suffering because imports are not coming in and markets are not functioning."

According to the UN, some 21 million Yemenis — 80 percent of the population — remain in need of assistance. Those who still have access to supplied markets report huge inflation in the cost of basic goods, with wheat flour doubling in price and cooking gas costing as much as 264 percent more than it did before the conflict. In many areas, there is insufficient fuel to run mills, pump water, and provide electricity for health facilities. That has exacerbated the country's existing water crisis; potable water is either unavailable or only supplied sporadically in 20 of Yemen's 22 governorates.

Riebl said coalition navies hold up practically every boat destined for a Yemeni port.

"The process it relatively unclear, so ships are discouraged from even venturing there, and those that do have huge delays getting their food into Yemen," he said.

Earlier this month, the UN's World Food Program said delays at the Red Sea port of Hodeida had caused it to discard a 3,000 metric ton shipment of wheat flour.

Since March 26, the UN has recorded 1,895 civilian deaths in Yemen, including at least 202 who died in the period between July 16 and 27. Earlier this month, the UN's human rights office said most of the casualties in recent weeks were caused by airstrikes, which have repeatedly hit markets and residential areas. Last Friday, just hours before the Saudi coalition announced the unilateral ceasefire, its jets struck two residential compounds at a power plant in the coastal city of Mokha. On Tuesday, Human Rights Watch, which recorded the deaths of at least 65 civilians, including 10 children, said the attack appeared to be a war crime. The group called for the UN human rights council to establish an international commission of inquiry — similar to the body formed after the war in Gaza last year — to investigate violations of international law in the Yemeni conflict.

The Saudi-declared humanitarian pause, which the Saudis said was requested by Yemeni president Abd Rabbuh Mansur Hadi, was widely viewed with skepticism given its unilateral nature and the failure of a previous pause that was brokered by the UN earlier this month, and which was then roundly ignored by all sides. In announcing the pause, the Saudi coalition said it reserved the right to resume strikes in response to any Houthi military movement.

On Tuesday, airstrikes hit targets to the north of the southern city of Aden, which in recent weeks has seen the pushback of Houthi and allied forces by a combination of local militias and Emirati Special Forces with the backing of coalition air and naval strikes. Al Qaeda in the Arabian Peninsula — the extremist group's affiliate in Yemen, which has not been targeted by coalition jets — was also reportedly involved in fighting against Houthi forces. After losing the city, the Houthis and their allies allegedly shelled a nearby town of Dar Saad on Sunday, killing almost 100 people — most of them civilians — according to Doctors Without Borders.

The Houthis hail from a Shia minority in Yemen's north that has long felt marginalized by successive Yemeni governments. During the 2000's former Yemeni president Ali Abdullah Saleh waged a series of wars against the group. Following his departure during the Arab Spring in 2012, Saleh began backing the Houthis, who grew increasingly assertive during Hadi's tenure. In March, during a Houthi offensive on Aden, Hadi fled to Riyadh, where he has since remained, ensconced among Saudi royals.

The Saudi coalition accuses Iran of backing the Houthis, and says the blockade is necessary to prevent Tehran from supplying their troops. In April, Riyadh announced it would meet the UN's entire $274 emergency "flash" appeal for Yemen. But the Saudis have yet to deliver the money, and the UN's overall request of $1.6 billion for Yemen remains only 15 percent funded.

On Tuesday, UN humanitarian chief Stephen O'Brien told the Security Council that UN humanitarian agencies had depleted their reserves "in expectation of the original Saudi pledge."

"They are using their own resources," he told VICE News after the briefing. "There is a limit, they don't have endless resources."

The UN and the Saudi government say the transfer of funds is being delayed as nine separate UN agencies, including the WFP and the UN's development program, attempt to broker Memoranda of Understanding with Riyadh. Aid workers who spoke with VICE News say after the April announcement, the Saudis insisted on certain restrictions in how the aid is delivered — for instance limiting it to areas not controlled by the Houthis. O'Brien has said that any restrictions based on geography are not acceptable to the UN.

"As the UN, we have to have this ability to retain where we send it," he told VICE News on Tuesday. "We have to meet vulnerability and need and retain our impartiality, that's the key.

Earlier on Tuesday, VICE News asked Saudi Arabia's ambassador, Abdallah Y. al-Mouallimi about the delay. After initially stating that the money "is being disbursed right now," Almoalimi clarified that the transfer was "pending the signature of memoranda of understanding with the various organizations. Some have signed, some have yet to sign. It depends on the speed by which the UN bureaucracy works."

The Saudi government, the ambassador said, was ready to provide the money "as soon as Mr. O'Brien's people come over the sign the memoranda."

Asked about Almoalimi's comments, O'Brien replied, "You can say you are waiting for signatures on the document that may not be the agreed document between the two parties. In which case you can say I'm waiting but the other side doesn't want to sign."

Riebl, meanwhile, said the conversation has to be expanded to include the blockade, otherwise the impact of any effort will be insufficient.

"It's impossible to meet the need," said Riebl. "The solution has to be a lifting of the blockade and as much commercial access as possible. Then humanitarian agencies can on top of that try to directly reach those populations that don't have access."

_Follow Samuel Oakford on Twitter: _@samueloakford

This story has been updated since it was initially published to include comments from UN humanitarian chief Stephen O'Brien and Saudi ambassador Abdallah Y. al-Mouallimi.