The Associated Press news agency quoted officials as saying that the new radar was flown into Israel last week along with some 120 American crew members and has been set up at the Nevatim air base in the Negev desert.
The system can pick up a ballistic missile shortly after launch. That will cut the response time of Israel's Arrow system, designed to intercept incoming missiles.
The officials spoke on condition of anonymity because the radar's arrival has not been officially made public. It was first reported in Defence News.
Defence co-operation
The Israeli military said on Sunday that it has "various forms" of co-operation with the US military but that "as a rule we do not detail the content" of the ties.
Questioned by the AFP news agency, a defence ministry spokesman said he did "not know about such a deployment".
A senior Pentagon official had said in late July that Robert Gates, the US defence secretary, agreed to explore deploying a powerful missile defence targeting radar in Israel.
"The idea here is to help Israel create a layered missile defence capability to protect it from all sorts of threats in the region, near and far," the official, who spoke on condition of anonymity, said.
Besides the radar, Gates also agreed to explore sharing missile early warning launch data, as well as US funding for two costly Israeli projects designed to counter short-range rockets and mortars, he said.
The official said deploying the X-band radar was a near-term proposition, and "all this is moving pretty quickly."
Land-based system
"We are going to station this land-based system there, and the Israelis would plug into it," the official said.
An X-band radar is a powerful phased array radar that can target the warhead of a long or medium range missile in space. The US has deployed one in Japan and plans to install a larger X-band radar in the Czech Republic.
The official linked the assistance to the US administration's push for progress on a roadmap for peace between Israel and the Palestinians.
But it appeared to be more directly related to Israel's concern about Iran's nuclear programme.