"The establishment of a new settlement or the expansion of an existing settlement would violate Israel's obligations under the road map," Gonzo Gallegos, a State Department spokesman, told reporters in Washington.
"The U.S. calls on Israel to meet its road map obligations and avoid taking steps that could be viewed as predetermining the outcome of final-status negotiations," he said. Washington was seeking an explanation from Israel, Gallegos added.
Israel agreed to the road map in 2003. It was devised by the United States, the United Nations, the European Union and Russia in an effort to guide Israel and the Palestinians into an agreement that estalishes a Palestinian state.
The European Union said earlier Wednesday it was deeply concerned about the plan, which would turn a former Israel Defense Forces base in the West Bank into a settlement for 30 settler families evacuated from Gaza last year.
A U.S.-backed Israeli-Palestinian peace plan known as the road map calls for a halt to settlement construction in the West Bank, land Palestinians want for a state.
"This development runs contrary to the commitment undertaken by Israel in the road map," Finland, which holds the rotating presidency of the EU, said in a statement.
"The Presidency of the European Union expresses its deep concern at the news that the Israeli government has authorized the construction of the Maskiot settlement in the West Bank.
"Such unilateral actions are also illegal under international law and threaten to render the two-state solution physically impossible to implement," it said.
The families planning to move to the former army base in Maskiot lived in two of the 21 settlements Israel dismantled in the Gaza Strip in 2005 under the disengagement plan promoted by former Prime Minister Ariel Sharon.
Some 8,500 settlers were pulled out of the Gaza Strip, along with IDF troops, after 38 years of control.
"The Presidency of the EU urges the parties to refrain from all activities that would endanger the cease-fire," Finland said. "Extending the ceasefire to the West Bank must be an urgent goal."
Some 260,000 settlers live in the West Bank, among 2.5 million Palestinians. The World Court has branded Israeli settlements on land captured in the 1967 Six Day War as illegal.
Defense Minister Amir Peretz on Tuesday authorized the repopulation of Maskiot, an abandoned Nahal infantry brigade settlement in the Jordan Valley.
Peretz approved the construction of 30 new homes in Maskiot that are intended for families evacuated from Shirat Hayam and other uprooted Gaza Strip settlements.
Shirat Hayam was one of the more ideological settlements among the Katif settlement bloc. It was founded in response to a Palestinian terrorist attack against a bus carrying children from the nearby settlement of Kfar Darom.
The proposal to relocate the families in Maskiot was approved in principle by the former defense minister Shaul Mofaz and the former prime minister Ariel Sharon.
Maskiot originally had belonged to the ultra-Orthodox branch of Nahal and currently houses a premilitary academy.
The area of Maskiot, along with that belonging to Beitar Ilit, Givat Ze'ev, and Oranim, was expanded in May. Of the four settlements, Maskiot is the only one on the eastern side of the West Bank separation fence.
It was initially reported that Peretz would suspend authorization to repopulate the settlement. Construction on the new homes will begin in the upcoming weeks, according to Jordan Valley Regional Council Chairman Dubi Tal.