The radar will team up with Israel's Arrow anti-missile missiles
The US has deployed long-range radar in Israel to give early warning of any missile strike by Iran, officials say.
The radar has been set up at an air base in the Negev desert and will be operated by 120 US military personnel.
Israeli reports say it is the first time US military personnel have been based in the country permanently.
Iran has threatened to "set Tel Aviv on fire" if Israel carries out its threat to attack Iran's nuclear programme which Tehran says is entirely peaceful.
Correspondents say the deployment in Israel of the US high-resolution radar system is another sign of the strength of defence ties between the two countries.
But they say it also cements Israel's technical dependency on its ally and therefore boosts Washington's power to veto unilateral Israeli action aimed at denying Iran access to nuclear weapons.
The radar, known as X-Band or FBX-T radar, is being stationed at the Nevatim base, in southern Israel, which regularly hosts joint air exercises between Israel and the US.
"This is a major upgrade in bilateral preparations for the threats facing Israel," said an official quoted by the Reuters news agency.
Forward Based X-Band Transportable Radar (FBX-T) made by Reytheon Co
Uses microwave region of the electromagnetic spectrum
High-resolution imaging improves target identification and discrimination.
The satellite-assisted system, built by Raytheon, is said to be capable of tracking objects as small as tennis balls more than 1,200 miles (2,000km) away.
It would work in conjunction with Arrow anti-missile missiles, developed jointly by the US and Israel, which are currently supported by a less advanced radar system.
Israeli military analyst Ephraim Kam is quoted by Associated Press saying the new system is an "important addition" to Israel's defence system.
He said the message to Iran was that the US is against any pre-emptive attack by Israel, but the system would allow Israel to better defend itself if it or the US did decide to strike.
The US, Israel and many of their allies believe that Iran's nuclear activity is a precursor to developing nuclear weapons.
Israel sees an Iranian nuclear weapons capability as an existential threat, and has threatened a pre-emptive strike against Iran's nuclear installations.
Iran has missiles which are capable of hitting Israel within a few minutes
According to recent reports, the US has told Israel that it will not support any military attack on Iran.
Iran says its nuclear work is a legitimate programme to ensure future energy supplies after its oil runs out.
On Saturday, the UN Security Council adopted resolution 1835 urging Tehran to suspend uranium enrichment and reaffirming existing sanctions.
Israel is widely believed to be a nuclear-armed state itself, although it has a policy of "nuclear ambiguity" and has not signed up to international nuclear non-proliferation agreements, thereby avoiding UN inspections.