Doubts have been raised since members of the Nigerian government said on Friday that they had secured a deal with Boko Haram to free the girls captured in April and to end hostilities.
In the aftermath of Friday's declaration by Nigeria's military and presidency, reports of attacks continue to emerge, casting further doubts on the credibility of the ceasefire claim.
A group of Nigerian elders has also said only one faction of Boko Haram had been involved in the deal.
"We can confirm reports that a ceasefire has been announced, (and) appears to have been put into place," State Department deputy spokeswoman Marie Harf said on Monday.
"We would welcome that ceasefire, call on all parties both to implement and maintain such a ceasefire, and hope that such a ceasefire would herald the return of peace to the northeast.
"This is a region that has had far too little of that."
But she said it was Washington's "understanding that the negotiations about a deal to release the girls continue".
With 80 US military personnel sent to neighbouring Chad for intelligence, surveillance and reconnaissance, the United States is the biggest foreign participant in the effort against the Islamic militants, Boko Haram, who have terrorised parts of northern Nigeria.
Washington has also deployed surveillance drones, spy planes and about 30 civilian and military specialists to support Nigeria's security forces.
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