"They've got a clandestine weapons program, which, combined with delivery systems, is a threat to stability," deputy State Department spokesman Adam Ereli said when asked about Iran's test earlier Wednesday of an upgraded version of its conventional medium-range Shahab-3 missile.
Ereli had no specific reaction to the test, reported by Iran's official IRNA news agency, but said Washington was troubled by recent developments, including Tehran's continued denials of US claims that it is using a civilian nuclear energy program to conceal atomic weapons development.
"We believe they're of concern and we are working with our international partners to address them," he said, referring to efforts by the International Atomic Energy Agency (IAEA) to hold Iran to commitments on its nuclear program.
The Shahab-3 is considered the mainstay of Iran's military technology and portrayed as purely defensive and dissuasive, but specifically as a weapon against arch-foe Israel, which two weeks ago tested its Arrow II anti-missile missile.
In the July 28 test of Israel's Arrow II missile, the Jewish state made it clear the improved anti-missile system was aimed squarely at fending off any attack by Iran.
Tehran fears Israel could strike its controversial nuclear program.
The Shahab-3 missile, whose name means "meteor" or "shooting star" in Farsi, is thought to be capable of carrying a 1,000-kilogramme (one-tonne) warhead at least 1,300 kilometers (800 miles), well within range of Israel.
It is believed to be derived from technology acquired from Pakistan and North Korea, though Iran's defense minister has denied any dealings with Pyongyang.