The council acted just days before national assembly and provincial council elections are to be held in Afghanistan on Sunday.
The Nato-led multinational force of 10,000 troops, formally known as the International Security Assistance Force (ISAF), is helping Afghan forces ensure the elections are not disrupted or marred by violence.
The elections mark the formal end to a four-year process of international support launched in Bonn after US-led forces overthrew the Taliban in late 2001. International donors are to meet in London in January to chart a new aid programme.
ISAF is now concentrated in the capital Kabul and Afghanistan's north and west. Nato hopes eventually to expand the force into the more violent south and east, now patrolled by a US-led forces of roughly 20,000 soldiers.
The ISAF mandate would have expired on 13 October if it had not been renewed.
"The situation in Afghanistan still constitutes a threat to international peace and security," the 15-nation council said in a resolution it adopted unanimously.
The resolution also urged UN members to contribute more troops, equipment and money to the international force.