The UN's health agency said just a trickle of aid had managed to get into Tigray, which is in the grip of a humanitarian crisis after a two-year conflict, AFP reported.
Restoring aid deliveries to Tigray was a key part of an agreement signed on November 2 to end a war that has killed untold numbers of people.
"That peace process has not yet resulted in the kinds of full access, unfettered access and the massive scale-up of medical and health assistance that the people of Tigray need," WHO emergencies chief Michael Ryan told a press conference.
"I remain cynical on that front because we've been a long time waiting to get access to these desperate people."
Tigray was isolated from the world for over a year, and faced severe shortages of medicines and limited access to electricity, banking and communications -- services that need restoring for relief logistics operations to function.
"It's really hard to plan a scale-up when at every moment you can have your ambitions curtailed," Ryan lamented.
"The UN system is really anxious to scale up our operations.
"We welcome any cessation of violence, any access that's given.
"But the people in Tigray are desperate. They've been years now without access to proper healthcare and nutrition and they need our help now. Not next week, not next month. Now."
He said some WHO staff had been able to go in, while a small fuel allocation might allow the organization to service a tiny percentage of the needs in the region.
WHO chief Tedros Adhanom Ghebreyesus welcomed the ceasefire, deemed a vital prerequisite for health, but urged that it be implemented in full.
"The need is massive," said Tedros, who is himself from Tigray.
He insisted that food aid and medical supplies should be delivered to civilians at all times during conflicts.
Tedros and Ryan both raised concerns for areas that are still under the control of troops from neighboring Eritrea.
The ceasefire makes no mention of the presence on Ethiopian soil or any possible withdrawal of Eritrean troops, who have backed Ethiopian Prime Minister Abiy Ahmed's forces and been accused of atrocities.
Last week the UN's World Food Program said aid deliveries into Tigray were "not matching the needs" of the stricken region.
WFP said an estimated 13.6 million people across Tigray and its neighboring regions of Amhara and Afar were dependent on humanitarian aid as a result of the war, which broke out in November 2020.
Tigray's authorities had been resisting central rule for months when Abiy accused their leadership of attacking federal army camps and sent troops into the region.