Jerusalem (CNN) -- Five members of an Israeli family were killed in the West Bank early Saturday morning in what the Israeli military is calling a terror attack.

According to a military spokeswoman, an intruder entered the Israeli settlement of Itamar near the northern West Bank city of Nablus around 1 a.m., made his way into a family home and killed two parents and their three children. Speaking to reporters at the scene of the crime, the head of Israeli military central command, Maj. Gen. Avi Mizrachi, called the killings "a brutal terror attack" that "was perpetrated by cowardly murderers."

Military units were searching the surrounding area for the assailant, the spokeswoman said. The military would not provide more details. But local media reported that the attack was made with a knife, and neighbors reported hearing shots fired as well.

Military spokesman Maj. Peter Lerner said more than one person may have carried out the crime.

Israeli Prime Minister Benjamin Netanyahu demanded that the Palestinian Authority assist in finding those responsible and "halt the incitement in the mosques and via the Palestinian Authority-controlled media."

He did not explicitly blame Palestinians for the deaths.

"This is a heinous act of a murder of a whole innocent family -- parents, children and baby -- while they were sleeping in their home on the Sabbath night," Netanyahu said. "Israel will not accept such a heinous murder and will act strongly to keep the safety of the lives of Israeli citizens and punish the murderers."

He added, "We all know, as do those who want to strike at us will know, that the future of the settlements will not be decided upon by terror."

Palestinian Authority Prime Minister Salam Fayyad denounced the killings.

"I say that in regards to what happened (Saturday) in Itamar settlement -- and the victims were a toddler and two children, the father and mother -- exactly as we have always said against violence ... we refuse this and condemn it," Fayyad said, according to his spokesman, Ghassan Khatib. "We refuse and condemn violence, whatever the reasons are or the perpetrators or targets or victims."

The White House condemned the slayings "in the strongest possible terms" and said the killers of the "heinous" act must be held "accountable."

"There is no possible justification for the killing of parents and children in their home," it said in a statement.

Secretary of State Hillary Clinton added that she was "shocked and deeply saddened" by the incident.

"The murderers must be found and brought to justice," she said, asking the Palestinian authorities "to assist in every way possible."

The United Nations, the European Union, the Russian Federation and United States also issued a joint statement condemning the murders. The four entities are members of the Quartet on the Middle East, an association involved in mediating the peace process between Israel and Palestine.

"The Quartet condemns in the strongest possible terms the violent murder of an Israeli family of five, including three young children, in the West Bank today," the statement read. "The Quartet offers their condolences to the loved ones of the victims and to the Israeli people."

"Attacks on any civilians are completely unacceptable in any circumstance," the statement continued. "The Quartet calls on those responsible to be brought to justice and welcomes the strong condemnation of this attack by President Abbas and the Palestinian leadership."