Israel is "deeply embedded" in Azerbaijan and can use its airbases bordering Iran to carry out an attack against Iranian nuclear installations, a media report said.
"The Israelis have bought an airfield," a senior US administration official was quoted as saying by Foreign Policy magazine.
The magazine quoting three diplomatic sources said that Baku has granted Jerusalem permission to use four abandoned, Soviet-era airfields, should the need arise.
In the event of an Israeli air strike on Iran's nuclear installations, access to Azeri airfields would rid Israeli fighter jets from the need to refuel in mid-flight, the report said.
United States intelligence officials are worried that Israel's military involvement in Azerbaijan would make it more difficult for Washington to reduce Israeli-Iranian tensions.
"Apparently now, military planners must prepare for a war scenario that would also involve the Caucasus," the report said.
Foreign Policy quoted an intelligence officer as saying that Washington is "watching what Iran does closely... but we're now watching what Israel is doing in Azerbaijan. And we're not happy about it".
Israel-Azerbaijan ties have been growing stronger over the past few months with the two signing a 1.6 billion USD (Rs 8320 cr) arms deal in February which will see Jerusalem supply Baku with sophisticated drones and missile-defence systems, the report said.
Washington believes that there is a covert aspect of the Israeli-Azeri alliance and that "the security cooperation between the two countries is heightening the risks of an Israeli strike on Iran".
Defence analyst David Isenberg said that "the ability to use Azeri airfields is a significant asset to any Israel strike".
In fact, he added, "being allowed access to Azeri airfields would be crucial," as Azerbaijan is strategically located on Iran's northern border.
The magazine noted that both Israel and Azerbaijan were asked to comment, but opted to stay mum.
Meanwhile, a new report commissioned by the US Congress said that an Israeli strike on Iran would only halt Tehran's nuclear project by six months, Ynetnews reported.
The report says that the wide dispersal of Iran's nuclear facilities would pose a major difficulty for Israel should it attempt to curb Tehran's nuke programme.
Researchers have said that the potential effect of a military strike on Iran's ability to acquire nuclear weapons is "unclear".
Moreover, US and Israel cannot pinpoint all of Iran's nuclear sites, they said.
