When talks between Serbia and the southern province's ethnic Albanians failed to resolve their dispute over the territory, the problem would be taken over by U.S. peacekeepers.
About 90 soldiers, all members of the National Guard from the U.S. state of Iowa, joined French peacekeepers at NATO's Camp Nothing Hill in a part of Kosovo that is dominated by members of the province's Serb minority. Most of Kosovo's people are ethnic Albanians.
The area has eluded United Nations control and defied locally elected institutions since Kosovo came under international control after the 1998-99 war.
Germany recently sent 500 additional troops to the province's north, boosting the force's presence ahead of Dec. 10 - the deadline for international mediators involved in the failed negotiations to report back to U.N. Secretary-General Ban Ki-moon.
NATO officials in Kosovo, however, said the deployment was a regular troop rotation and was not linked to Kosovo's status talks, in which mediators failed to coax a compromise between Kosovo's demand for outright independence and Serbia's refusal to let it fully break away.
The chief U.S. envoy to the talks, Frank Wisner, has warned the region is up for "a very difficult time" as the failure to reach an agreement has increased the likelihood that the breakaway province will unilaterally declare statehood sometime in the coming months.
NATO's commander in Kosovo brushed aside security threats, saying the 16,000-strong NATO-led force known as KFOR was ready to respond.
"KFOR is everywhere and is ready to face any kind of threat at any time, coming from anywhere," French Lt. Gen. Xavier Bout de Marnhac told reporters after a brief ceremony at the military base.
The U.S. peacekeepers will be stationed in northern Kosovo for two months to conduct foot patrols in Serb-populated villages.