A toddler writhed in pain on a hospital bed in the Yemeni capital of Sanaa, a tube running through her nose, her eyes swelled shut and lashes wet with tears.

In the hallway outside, doctor Mohammed al-Ayzari was alert, but his face looked worn from apparent exhaustion.

"The malnutrition cases are up more than ever before," he told AFP.

"There is an acute shortage of medical supplies and laboratory materials."

After years of war that has seen a Saudi-led coalition battling Iran-backed Huthi rebels impose a blockade on the country, the UN has already warned that seven million people in Yemen are on the verge of starvation.

But now a decision by Riyadh and its allies to tighten the screws on all land, sea and air borders in response to a Huthi missile attack has sent prices spiralling further -- and ratcheted up fears of a looming famine.

At a shuttered gas station in Sanaa a family waited hope against hope to fill their three battered canisters -- used for cooking in most of the Arab world.

The mother, dressed in black, slumped on the pavement as her husband stood glumly with their little girl and two boys.

"We've been here for almost one week, waiting for fuel," another man, Fuad Al-Harazi, told AFP.

"Every day they say the fuel truck is here but that isn't true."

- 'Can't survive' -

Prices have soared in Sanaa since the Saudi-led coalition upped the pressure on Yemen a week ago, the UN's Office for the Coordination of Humanitarian Affairs says.

The cost of fuel has gone up by nearly two-thirds, the price of trucked water has increased by 133 percent and bus fares have doubled or even tripled.

Cars could still be seen on the roads around the rebel-held city, but many stations were cordoned off with plastic ribbon.

Standing by yet more closed petrol pumps, Ameen Mohammed vented his anger at the coalition.

"We are besieged from abroad, we are besieged at home. We don’t have gas to cook. People are dying in their houses because of the gas shortages," the young, clean-shaven man said.

"Why are they besieging Yemen? What do they get out of it?" he demanded.

On a corner where street vendors were selling cheap household items, neatly dressed government employee Amer Ali echoed the despair.

"The higher price of fuel is making the food prices go way up," he said.

"The average person can't survive."

- 'Unimaginable' suffering -

Saudi Arabia and its allies intervened in Yemen's conflict in March 2015 with the stated aim of rolling back the Iran-backed Huthis and restoring the government of President Abedrabbo Mansour Hadi to power.

More than two and a half years later,thousands have died in the conflict and the rebels still control Sanaa and much of northern Yemen.

UN aid chief Mark Lowcock has told the Security Council that unless the Saudi-led blockade is lifted, Yemen will face "the largest famine the world has seen for many decades, with millions of victims."

Saudi Arabia's ambassador to the UN Abdallah al-Mouallimi on Monday sought to allay concerns over the latest move, noting ports in Yemen's government-controlled areas were being reopened.

But Riyadh is insisting that there must be tougher checks at rebel-held ports before they can open -- a demand the UN contends is proving catastrophic.

Aid officials point out that Yemen's government-held bastion of Aden lacks the capacity to ensure a steady flow of distribution to the millions who depend on it.

Rebel-held Hodeida port, on the other hand, is closest to the majority of people in need.

The UN's aid coordinator in Yemen, Jamie McGoldrick, maintained Tuesday that keeping ports closed is unacceptable and that the Saudi-led coalition could use current inspection mechanisms to let them work.

"We can't have those ports closed or those airports closed while we wait for discussions on new (inspection) mandates to go ahead," he said.

"The humanitarian impact of what is happening here right now is unimaginable."