The US Department of State, speaking after Iran declared it had ballistic missiles with a range of 2000km and could produce ones with a greater range, said Washington would continue to work with other nations to address the concerns.
"The United States has had and continues to have serious concerns about Iran's missile programmes," deputy spokesman Adam Ereli said in Washington on Tuesday.
"We view Iran's efforts to further develop its missile capabilities as a threat to the region and to the United States' interests and all the more so in light of its ongoing nuclear programme," he said, repeating US allegations that Iran is actively seeking atomic weaponry.
Rafsanjani warned the US against
attempting to attack Iran
Tehran has denied the US charges on nuclear weapons.
Earlier on Tuesday former Iranian president Akbar Hashemi Rafsanjani told a conference on space and stable national security in Tehran that Iran now had "the power to send our missiles up to 2000km".
The range puts parts of Europe within reach for the first time. Military experts had earlier placed Iran's missile range at 1300km, which would allow it to strike anywhere in Israel.
"If the Americans attack Iran, the world will change ... they will not dare to make such a mistake," Rafsanjani said.
In recent months, Iranian officials have frequently trumpeted their ability to strike back at any aggressor, and in August announced they had successfully tested an upgraded version of the medium-range Shahab-3 missile.
Iran's missiles reportedly can now
Military experts say the unmodified Shahab-3 has a range of 1300km. Shahab means meteor in Persian.
While Iran has had missiles capable of reaching Israel for some time, Israeli officials said the longer 2000km range was more significant for Europe than for Israel.
"We are well prepared to defend the state of Israel. The Iranians will have to think twice before using these kinds of weapons," a senior Israeli government official said.
Iranian Defence Minister Ali Shamkhani in September said that a new "strategic missile" had recently been delivered to the armed forces, but did not give its range.
Israel has long accused Iran of working on a long-range missile, the Shahab-4, which would be able to reach Europe, but Iran denies plans to build a Shahab-4 missile.
"If the Americans attack Iran, the world will change"
Akbar Hashemi Rafsanjani,
However, Tehran recently announced plans to launch its own satellite into space next year. Military experts say a satellite launch rocket could easily be adapted for military purposes.
"We are very happy that our defence ministry ... will take us to the stage that we are able to use independent satellite technology in the fields of building, launching, positioning and receiving," Rafsanjani said.