WASHINGTON - The United States said on Tuesday that it will do whatever is necessary to maintain the openness of the Strait of Hormuz.

State Department spokeswoman Victoria Nuland called the strait "an international waterway" that needs to stay open to vessels of all states having transit passage rights.

"These rights apply to warships, as well as merchant ships and vessels," she told reporters at a regular news briefing, saying " Any attempt by Iran to close the strait or to require vessels to obtain Iranian consent would be inconsistent with international law and not recognized by the United States."

"We have over years and decades made clear that we intend to do what is necessary to maintain the openness of the strait," she added.

As the United States and the European Union moved in recent days to stifle the exports of Iran's crude oil, the lifeline of the Islamic republic's economy, to force a changed course over its nuclear program, some Iranian lawmakers are preparing a bill calling for the closure of the Hormuz, one of the world's most critical oil route.

Nuland also expressed "deep concern" about Iran's "continued" missile development activities, calling them "not a positive development."

Iran's elite Islamic Revolution Guards Corps on Tuesday test- fired a diversity of missiles and tested its unmanned aerial vehicle bombers during a drill.

The official IRNA news agency said the short, medium and long range missiles of the unit "successfully" targeted the hypothesized sites of an imaginary enemy in the central desert area of Iran.

Iran, following three rounds of negotiations over its uranium enrichment program since mid-April, started on Tuesday in Istanbul, Turkey a new round of nuclear talks at the level of technical experts, trying to explore common ground for full-fledged talks after previous meetings failed to produce any breakthrough.

Iran insists on the peaceful nature of its nuclear activities, which the Western countries doubt are being used to produce nuclear weapons.