The process moved ahead when Russia "lifted its hold placed on the Dutch proposal" to include the six people on the sanctions blacklist, a diplomat said, adding that the sanctions take effect immediately.
"Last fall, images of migrants being sold as slaves in Libya shocked our conscience, and the Security Council vowed to take action," said the US envoy to the UN, Nikki Haley.
"Today's sanctions send a strong message that the international community is united in seeking accountability for perpetrators of human trafficking and smuggling. There is no place in our world for such abuses of human rights and human dignity."
The sanctions -- a global travel ban and an assets freeze -- target two Eritrean nationals described as top operators in transnational smuggling networks and four Libyans including the head of a regional coast guard unit.
When the Security Council first mulled sanctions, Russia asked for additional information "to better understand the value of the said proposal, how efficient it might be if approved," according to a letter obtained by AFP.
Russia called for a meeting to examine the evidence on the involvement of the six individuals and noted that the criminal networks "stretch to many European countries and the United States."
The sanctions blacklist was presented May 1 and Russia put a hold on the request a week later.
Among the six now sanctioned is Ermias Ghermay of Eritrea, described as a leader of a network responsible for "trafficking and smuggling tens of thousands of migrants" from the Horn of Africa to the coast of Libya and onwards to Europe and the United States, according to the original sanctions request obtained by AFP.
The other five are Fitiwi Abdelrazak of Eritrea, Libyan militia leader Ahmad Oumar al-Dabbashi, Libyan Musab Abu-Qarin, described as a "central actor" in migrant smuggling in the coastal area of Sabratha, Libyan Mohammed Kachlaf, head of the Shuhada al Nasr brigade in Zawiya, western Libya and finally, Abd al Rahman al-Milad, who heads the Libyan coast guard in Zawiya.
At the end of 2017, US network CNN aired footage of Africans sold as slaves in Libya, sparking outrage from political leaders and street protests in African and European capitals.