NAIROBI, Kenya — Compared with the trillions of dollars flowing through the international financial system every day, the estimated $1.3 billion sent each year from small storefronts in Somali neighborhoods in places like London and Minneapolis to families back home in the Horn of Africa is a mere droplet.

In a country where 40 percent of the population depends on remittances from relatives abroad, however, these transactions are a lifeline. And because Somalia has little in the way of financial infrastructure, the money transfer companies that make them possible play a central role in keeping the financial ties to the diaspora open, a few hundred dollars at a time.

But in an echo of politically charged fights over aid that have vexed Somalia before, the same money transfer services that help keep children in school and elder relatives off the streets also can be used to send money to help finance dangerous groups like the Shabab, the feared Islamist militants who once controlled much of the country.

Humanitarian groups, politicians and Somalis themselves are now sounding the alarm over plans by the British bank Barclays to suspend the accounts of a number of money transfer companies used to send money to developing countries — rather than risk a run-in with regulators over potentially abetting the financing of terrorists or money laundering.