Plans to ship as many as one million doses of cholera vaccine to Yemen are likely to be shelved over security, access and logistical challenges, UN officials said.
The UN aid co-ordination agency said Yemen's suspected cholera caseload has surged past 313,000 and caused over 1,700 deaths, making it the world's largest outbreak.
War has crippled Yemen's health system, depleted access to safe drinking water and put millions on the brink of famine.
Citing such complexities, spokesman Christian Lindmeier of the World Health Organisation (WHO) said that shipping vaccines "has to make sense", and that they could be re-routed to places that "might need them more urgently", such as some African countries.
The announcement from the UN officials comes a day before the WHO's new director-general, Tedros Adhanom Ghebreyesus, is expected to address the UN Security Council by video-conference about Yemen's cholera crisis.
Last month, following a request from Yemen's internationally recognised government, the WHO and several key partners agreed to send one million doses of vaccine, the largest of its kind since one million doses were sent to Haiti after Hurricane Matthew last autumn.
Mr Lindmeier said 500,000 cholera vaccine doses are currently waiting in Djibouti for possible delivery to Yemen, and that Yemen's government has final say whether they are actually sent.
The Yemeni government is allied with a Saudi-led coalition that is battling Shiite Houthi rebels, who control the capital Sanaa and much of northern Yemen.
Cholera vaccine rollouts are not easy even in more peaceful situations.
The vaccines have to be kept in cold storage, and patients should receive a follow-up vaccination after the first one.
In Yemen, where cholera has now reached all 21 governorates, the vaccines have to be targeted to those areas most susceptible to new outbreaks.
That is hard in Yemen, which has remote areas and conflict zones that have sliced up the country.
Jamie McGoldrick, the Yemen chief for the UN Office for the Co-ordination of Humanitarian Affairs, said the plan to ship vaccines was designed as a "preventive intervention", but in some cases, the impact would be "less than optimal" by the time the vaccines would arrive.
Speaking by phone to reporters in Geneva, Mr McGoldrick said the trend line of cases in areas that have gotten help fighting cholera has been "stable, or (has) shown signs of decline".
"However, there are many areas in the country where the trend line is moving up, and those areas are, as you would imagine, the most remote," or behind conflict lines, he added.
He said Yemen now has 313,538 suspected cases and 1,732 deaths caused by cholera in an outbreak that was first recorded in late March.