Kenya is urging the UN to list Al Shabab under the same sanctions as Al Qaeda and ISIS, but foreign donors say the move could leave millions in drought-stricken Somalia without aid.
The proposed listing, which could take effect on Thursday, comes at a critical time in Somalia, where 2.2 million people, or nearly 18 per cent of the population, face severe hunger.
Al Shabab is already targeted under broader sanctions imposed by the United Nations on Somalia, which is heavily aid-dependent after three decades of conflict and economic ruin.
UN agencies and humanitarian organisations are exempt from these sanctions, which enables them to deliver urgent aid without prosecution when they venture into territory controlled by Al Shabab.
But Kenya wants to tighten the screws on the militant group after several deadly attacks on its soil, and the sanctions regime it proposes would remove that safeguard.
"A measure like this will have the effect of criminalising humanitarian aid," Eric Schwartz, president of Refugees International, told AFP.
"Any measure that would impact the current provision of aid would have extremely serious and substantial implications."
If no member state objects before August 29 the Al Shabab listing under Security Council resolution 1267 will take immediate effect.
Hundreds of millions of aid dollars for Somalia will then be thrown into doubt.
In some cases, foreign donors said they may need to freeze payments for up to a year as they consider how to comply with the new sanctions, said an aid source in New York liaising with the UN on the issue.
"We would be operating in a huge grey area. As humanitarian actors, we would have this huge dilemma of carrying on providing aid, or we stop altogether," the official working for a large global charity told AFP on condition of anonymity.
Another concern is that banks, fearing repercussions, could limit financial services to humanitarian agencies working in Somalia, a process known as "de-risking" that makes it difficult to transfer money and fund programmes.
The looming deadline has sparked a flurry of lobbying at Security Council headquarters in New York.
Earlier this month, 20 former ambassadors, national security and humanitarian officials wrote to US Secretary of State Mike Pompeo, Treasury Secretary Steven Mnuchin and USAID administrator Mark Green urging them to reject the Kenyan proposal.
Their decision "could either help sustain critical and life-saving relief to Somalia, or exacerbate the dire humanitarian situation and puts hundreds of thousands if not millions of people at grave risk", the August 7 letter said.
"They [humanitarian groups] have worked carefully to develop systems to reach people while limiting aid diversion" to Al Shabab, it said.
Exemptions from sanctions are extremely rare, the International Peace Institute said in a June paper.
Kenya sees Al Shabab as no different than extremist groups elsewhere and believes sanctions are a way to blunt their violence.
On July 13, after 26 people were killed in Somalia's south, Kenyan Foreign Minister Monica Juma said "this attack was another reminder to the international community of the imperative" to list Al Shabab under resolution 1267.
The foreign ministry did not reply to queries before publication.
Al Shabab violence has surged in 2019 with atrocities in Somalia and Kenya, including an attack in Nairobi in January that left 21 people dead.
Kenya has applied considerable diplomatic leverage on its allies, notably lobbying the EU in May to list Al Shabab as a "terrorism organisation".
The timing could not be worse for Somalia, which is facing another hunger crisis after the rainy season failed this year.
The country is prone to devastating, frequent droughts. A famine between October 2010 and April 2013 killed almost 258,000 people, more than half of them children under five.
Somalia has accused its larger neighbour of meddling that could worsen its domestic problems and have repercussions for regional stability.
Somalia's envoy to the UN, Abukar Dahir Osman, told the Security Council on August 21 that impeding aid efforts "will play into the Shabab's narrative and self-image as a de facto government in areas where state reach is limited".
He repeated the government's "condemnation of any interference in the internal affairs of Somalia".
Kenya's campaign on Al Shabab comes as it spars with Somalia on several diplomatic fronts including a battle over its marine borders, said Matt Bryden, director of Nairobi-based think tank Sahan.
"There's a range of thorny issues between the two governments that have strained their relations to the breaking point, and I think Kenya's move must be understood in that wider context," Mr Bryden said.