Former rebels in the Central African Republic killed almost 1,000 people in a two-day rampage this month, double an earlier UN estimate, Amnesty International has said.
The group says war crimes are being committed in CAR.
In a separate report, Human Rights Watch is calling on the UN to send a peacekeeping mission to CAR.
The mostly Muslim fighters ousted President Francois Bozize in March, sparking sectarian conflict.
Rebel leader Michel Djotodia became the Christian majority country's first Muslim leader and is now interim president.
The US ambassador to the UN, Samantha Power, arrived in the CAR on Thursday for talks with Mr Djotodia and religious leaders.
Urgent action was needed to end the "vicious violence", she said, on the eve of her visit.
"The authorities and the opinion leaders have to make difficult choices to help stop the violence, bring their people together, begin a process of reconciliation," she said.
In its report, Amnesty said former Seleka rebels killed nearly 1,000 people in the capital, Bangui, in revenge for deadly Christian militia attacks.
The death toll was much higher than earlier UN estimates, which spoke of 450 killed in Bangui and another 150 elsewhere in the country.
The attacks came after Christian militias known as "anti-balaka" (anti-machete) went door-to-door in some parts of Bangui "and killed approximately 60 Muslim men", Amnesty said.
"The de-facto government forces, known as ex-Seleka, retaliated on a larger scale against Christians in the wake of the attack, killing nearly 1,000 men over a two-day period and systematically looting civilian homes. A small number of women and children were also killed," the report said.
Amnesty says civilians are being killed on a daily basis in Bangui, despite the presence of French and African Union troops.
Christian Mukosa, Amnesty's Central Africa expert, said the information gathered "has left no room for doubt that war crimes and crimes against humanity are being committed by all parties to the conflict."
In its report, Human Rights Watch (HRW) said that in the northern town of Bossangoa, Christian militias attacked Muslim communities, cutting the throats of children while forcing parents to watch.
Muslim groups retaliated, setting fire to Christian homes and killing their occupants with the apparent approval of military commanders who were present, Human Rights Watch says.
"The brutal killings in the Central African Republic are creating a cycle of murder and reprisal that threatens to spin out of control," said Peter Bouckaert, author of the HRW report.
"The UN Security Council needs to act quickly to bring this evolving catastrophe to a halt."
Ms Power said she and President Barack Obama were "deeply disturbed" by the killings.
"This world has been witness to great atrocities before and each situation has its own unique aspects. Direct comparisons with other crises, past crises, are inevitably flawed," she said.
"But it is worth noting that Somalia taught us what can happen in a failed state, and Rwanda showed us what can occur in a deeply divided one."
Earlier this month, President Djotodia told French radio he was ready to speak to the leaders of Christian militias to resolve the worsening conflict.
His comments come after the African Union authorised increasing the number of troops in CAR to 6,000.
The AU troops, along with some 1,600 from France, are attempting to disarm militia fighters.