Iran is not yet "nuclear capable" and the U.S. government has not concluded that it is inevitable that Tehran will get the bomb, Pentagon chief Robert Gates said in remarks aired on Sunday.

"It is our judgment ... they are not nuclear capable, not yet," Gates, the U.S. defense secretary, said on NBC's Meet the Press.

Asked if the U.S. government had concluded this was inevitable, Gates said, "No. We have not ... drawn that conclusion at all, and in fact we are doing everything we can to try and keep Iran from developing nuclear weapons."

However, he added that "they [the Iranians] are continuing to make progress on these [nuclear] programs. It is going slower than they anticipated but they are moving in that direction."

U.S. President Barack Obama is pressing other global powers to agree to a fourth round of United Nations sanctions against Iran over its refusal to halt nuclear work that the West suspects is aimed at making bombs, a charge Iran denies.

But some critics of Obama's attempts to engage Iran have said they fear his administration may be preparing to shift from a strategy of keeping Iran from getting the bomb to a strategy of containing a nuclear-armed Iran.

"We are probably going to get another UN Security Council resolution" of sanctions on Iran, Gates told NBC.

Gates added that the United States and other countries will continue trying to convince the Iranians that they are "headed down the wrong path" by pressuring Iran with sanctions as well as more missile defense and other military cooperation in the Gulf region.

"At the end of the day, what has to happen is that the Iranian government has to decide that its own security is better served by not having nuclear weapons than by having them," Gates said.

Earlier Sunday, Iran's foreign ministry said that the Islamic republic will lodge a complaint with the UN about what it sees as Obama's threat to attack it with nuclear weapons.

Obama made clear last week that Iran and North Korea were excluded from new limits on the use of U.S. atomic weapons - something Tehran interpreted as a threat from a long-standing adversary to attack it with nuclear bombs.

"The recent statement by the U.S. president ... implicitly intimidates the Iranian nation with the deployment of nuclear arms," Iran's Supreme Leader Ayatollah Ali Khamenei said in a televised meeting with military and security officials.

"This statement is very strange and the world should not ignore it since in the 21st century, which is the era of support for human rights and campaigning against terrorism, the head of a country is threatening to use nuclear war."

Foreign ministry spokesman Ramin Mehmanparast told the semi-official Fars news agency Iran would lodge a formal complaint with the UN, a move backed by a letter signed by 255 of Iran's 290 members of parliament.

Reflecting fears of attack on its nuclear sites from the United States or its closest Middle East ally Israel, the defense ministry said Iran had started producing a prototype of an advanced anti-aircraft missile system.

"The Mersad air defense system ... is able to destroy modern aircraft at low and medium range altitude," the ISNA news agency on Sunday quoted Defense Minister Ahmad Vahidi as saying.

"The mass production of this product has begun and in the course of the current year a large number of them will be delivered to the armed forces," he said.

While Iran hopes the development of its own system will make it more self-sufficient in weapons defense, it is also urging Russia to resist Western pressure not to deliver the S-300 missile defense system it has ordered.

On Friday, President Mahmoud Ahmadinejad said Iran's nuclear program was "irreversible" despite limits on importing foreign technology and the threat of new sanctions, and he unveiled a prototype of an improved centrifuge which would enrich uranium faster than existing models.

Western analysts say Iran has exaggerated progress in the past to bolster domestic pride about its nuclear program and to improve its bargaining position with major powers.

The head of Iran's Atomic Energy Organization played down the idea that Iran faced big technical hurdles.

"Iran's nuclear issue is not a technical issue ... we are not in a hurry. Second generation centrifuges will be mass produced in the next few months ... in a year we will have prototype cascades of the third generation," Ali Akbar Salehi told ISNA.