"We have some 21 million people needing assistance and seven million of those are in famine-like conditions and rely completely on food aid," U.N. humanitarian coordinator for Yemen Jamie McGoldrick said.
"The continued closure by the Saudi-led coalition of critical seaports and airports is aggravating an already dire humanitarian situation. I think it poses a critical threat to the lives of millions who are already struggling to survive."
McGoldrick was speaking to reporters in Geneva by phone from Amman, because he said flights into Sanaa were blocked.
The risk of foreign aid work, especially for young people, has again been thrust into the national spotlight after the death of 26-year-old Kayla Mueller.
Mueller, a foreign aid worker, was confirmed dead Feb. 10 after being taken hostage by Islamic extremists in 2013 in Syria.
Even as aid organizations have improved security protocols over the past several years, workers can be placed in war-torn areas where safety cannot be guaranteed, said Abby Stoddard of Humanitarian Outcomes, a research and policy group for humanitarian agencies.
And those who feel compelled to take part in easing human suffering abroad may put safety second.
As the number of Muslims in Bangui, the Central African Republic capital, dwindles to an estimated 900, the head of a global alliance of churches has urged tackling the conflict from a political rather than a religious angle if the Muslim exodus is to be reversed.
The alliance is one of the agencies providing humanitarian assistance in the country, where chaos erupted last year after the mainly Muslim rebels toppled the government.
The Seleka rebels looted, raped, and killed mainly Christian civilians, prompting the formation of an equally brutal pro-Christian anti-Balaka (anti-machete) militia.
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