Panasonic, the exclusive supplier of batteries to Tesla, has decided to halt buying cobalt from a Canadian company after Reuters raised questions about its provenance.

Cobalt is a crucial element in the manufacture of lithium-ion batteries, which are found in Tesla cars, among most other consumer electronics.

The news outlet, citing anonymous sources, said that "some of the cobalt" in Tesla's batteries contain cobalt mined in Cuba by Sherritt International, based in Toronto.

Despite a recent thaw in US-Cuba relations, the embargo remains in place—so "most transactions between the United States... and Cuba continue to be prohibited," according to the Treasury Department.

Most cobalt is mined in the Democratic Republic of Congo amid horrendous labor conditions. The Washington Post reported in 2016 that electric cars typically have "20 to 30 pounds" worth of cobalt in them.

"No one knows exactly how many children work in Congo's mining industry," the Post reported. "UNICEF in 2012 estimated that 40,000 boys and girls do so in the country's south. A 2007 study funded by the US Agency for International Development found 4,000 children worked at mining sites in Kolwezi alone."

Companies that want to avoid the DRC commonly turn to Russia, Canada, Australia, or Cuba.

Cobalt mined in Canada or Australia likely costs notably more than cobalt mined in most other countries.

Panasonic, which did not immediately respond to Ars' request for comment, told Reuters that it did not know how much Cuban cobalt was in the batteries "due to co-mingling." But the company suggested that it was ending its relationship with Sherritt as a result.

Tesla CEO Elon Musk has said previously that the company only uses "less than 3 percent cobalt" at present and plans to phase it out.

We use less than 3% cobalt in our batteries & will use none in next gen — Elon Musk (@elonmusk) June 13, 2018

Panasonic, Sherritt International, and Tesla did not responded to Ars' request for comment.