The United States voiced concern Thursday over cooperation between Syria and Iran after reports Tehran had sent a radar system that would boost defenses against Israel.

The Wall Street Journal, quoting anonymous Israeli and US officials, reported that Iran last year sent the sophisticated radar that could help the Islamic republic detect an Israeli strike on its nuclear facilities.

State Department spokesman Philip Crowley said it was "hard" for the United States to determine whether such a transfer had taken place, but added: "We have concerns about the relationship between Iran and Syria."

"We don't believe that Iran's designs for the region are in Syria's best interest," Crowley told reporters.

While acknowledging that all countries "have the right to protect themselves," the spokesman said the reported radar delivery would be of concern due to Syria's relationship with the Lebanese Shiite militia Hezbollah.

"Our concern, obviously, in the case with Syria is the transfer of technology to Hezbollah," Crowley said, noting the issue was "something that we do raise with Syria in our periodic discussions with them."

President Barack Obama's administration has been trying to engage Syria and has asked the Senate to approve the first US ambassador to Damascus in five years.

The appointment has proven controversial in Washington, especially after Israeli President Shimon Peres said this year that Syria was supplying Hezbollah with Scud missiles that could cause major damage on Israeli cities.

But Syria has denied transferring Scuds to Hezbollah and the United States has not publicly confirmed the allegations.

The Pentagon declined comment on the Journal's report.

A senior US official said that even if Iran sent the radar system, it was unclear if the transfer broke any international resolutions as Syria is under only some military sanctions.

"Radars are by definition a defensive system by themselves," the official said on condition of anonymity.

"The real issue is what are they going to do with that and are those developments stabilizing or destabilizing."

Hezbollah, which has close ties with Iran, showered rockets on Israel in 2006, triggering a devastating war.

Israel and Western nations suspect Iran is developing nuclear weapons, although the clerical regime says its program is only for peaceful purposes.