The head of Iran's elite Revolutionary Guard Corps has warned the U.S. against attacking the Islamic Republic, saying if it did, Washington would be "stuck in a quagmire" worse than Iraq or Afghanistan.

Revolutionary Guards commander Mohammad Ali Jafari said his forces were prepared to strike back with a "crushing response" if attacked, according to comments carried Wednesday by the semiofficial Fars news agency.

"If enemies prove to be naive enough to invade Iran, they will be slapped hard," Fars, which is deemed close to the Guards, quoted Jafari as saying.

"The enemy knows that if it attacks Iran, it will certainly get stuck in a quagmire deeper than Iraq and Afghanistan and will be defeated," he added.

His comments come a day after President Mahmoud Ahmadinejad shrugged off new U.S. sanctions imposed last week that ban dealings with companies connected to the Guards.

Tensions are high between Iran and the U.S. over allegations that Tehran is secretly trying to develop nuclear weapons and supplying insurgents in Iraq with deadly weapons that kill American troops. Iran denies both claims.

Concerns have been mounting in recent months that the United States might attack to prevent Iran from developing atomic bombs. The U.S. has said it is trying to resolve its disputes with Iran diplomatically but also says it has not ruled out any options.

Speaking in Paris on Wednesday, U.S. Undersecretary of State Nicholas Burns said military action against Iran was "not inevitable, and not desirable" and suggested that tougher penalties would be a way to avoid it.

Washington is pushing for a third round of U.N. Security Council sanctions against Iran over its refusal to suspend uranium enrichment, a process that can be used to produce fuel for both nuclear energy and nuclear weapons. The Security Council has already passed two rounds of sanctions against Iran, but Russia and China have resisted a third set.

Burns was in France to urge European countries to slap new sanctions on Iran. But he also faulted China and Russia, which have economic interests in Iran, for stalling progress toward the third set of sanctions. He said China was increasing its trade with Iran, "which of course is a very negative development."

Underlining the rift between Russia and the U.S., the Russian Foreign Ministry said Wednesday it disagreed with a U.S. assumption that Iran is an enemy that must be countered with missile defense sites in Europe.

The statement from ministry spokesman Mikhail Kamynin indicated Russia remains strongly opposed to Washington's missile defense plans, despite recent U.S. attempts to assuage Russian concerns.

"A serious problem for us is that the U.S. plans are based on dogmatic logic that Iran is an enemy," Kamynin said. "We don't support plans for a 'holy alliance' against that country."

Russian President Vladimir Putin in the past has scoffed at U.S. claims that missile defense sites in Europe are necessary to counter a prospective threat from Iran, saying the shield would threaten Russia's nuclear deterrent.

Putin also said there was no proof that Iran was seeking to build nuclear weapons. He visited Iran earlier this month and reaffirmed that Russia would complete construction of Iran's first nuclear power plant.

The International Atomic Energy Agency, the U.N. nuclear watchdog, began a third, final round of talks in Tehran earlier this week aimed at resolving remaining questions on centrifuges that are used to enrich uranium. Those talks will provide a basis for an important progress report on Iran by IAEA chief Mohamed ElBaradei planned for mid-November.

Iran's U.N. Ambassador Mahammed Khazaee sent a letter to U.N. Secretary-General Ban Ki-moon on Wednesday, saying the U.S. economic sanctions against Iran were "a means of exerting pressure on a sovereign country for purely politically motivated and ill-intended policies."

He said that "Iran has demonstrated the purely peaceful nature of its nuclear program through complete transparency and full cooperation with the IAEA."

He also dismissed allegations that Iran supports terrorism.

Iran "considers the current U.S. warmongering as a threat not just against Iran but rather the whole world community," he said.