The Obama administration had opposed the S-300 sale because the system is highly effective against aircraft and some missiles. The CIA has said the S-300 missiles, which have been contracted by Tehran but not delivered, will be used to defend Iranian nuclear facilities.
A key provision in the resolution made public this week states that all U.N. member states will agree to block sales or transfers of weapons. It lists tanks, armored vehicles, artillery, combat aircraft, warships and "missiles or missile systems as defined for the purpose of the United Nations Register of Conventional Arms."
A close reading of the missile section of the register defines those included in the ban as missiles and launchers for guided rockets, and ballistic and cruise missiles, and missile-equipped remotely piloted vehicles. However, the register states that the missile system category "does not include ground-to-air missiles," such as anti-aircraft missiles and anti-missile interceptors like the S-300.
According to a CIA report to Congress on arms proliferation made public in April, Iran "continues to seek the advanced long-range S-300 air defense system from Russia."
"We judge likely deployment locations for this system include Iran's nuclear facilities," the report said.
Asked about S-300s, a senior State Department official said the draft "would not impose a legally binding obligation not to transfer S-300 to Iran" since the register does not cover defensive missiles.
However, the official said the draft resolution calls for U.N. members to show "vigilance and restraint" on arms sales, something "Russia has already demonstrated in its failure to deliver the system thus far despite its contractual obligation to do so."
The official noted that the arms embargo Moscow has accepted in the draft would block many other weapons systems that Russia has supplied to Iran and thus curb future transfers.
Mark Kornblau, a spokesman for the U.S. mission to the United Nations, had no comment.
Yevgeni Khorishko, a Russian Embassy spokesman, said his government is aware that the draft resolution does not ban sales of air-defense systems. "The S-300s is not prohibited," he said. "It is not on the list of prohibited items."
Mr. Khorishko said that for unspecific "technical reasons" the S-300 contract will not be implemented at this time.
It could not be learned why the administration did not ban air-defense missiles. The New York Times, quoting a U.N. diplomat, reported Wednesday that Russia opposed a total arms ban on Iran and that a final compromise on the sanctions resolution included a telephone call from President Obama to Russian President Dmitry Medvedev.
John R. Bolton, a former U.S. ambassador to the United Nations, criticized the Obama administration for not closing the sanctions resolution loophole on the S-300, calling it "diplomatic malpractice."
"It is further evidence that the Obama administration is preparing to accept a nuclear Iran," Mr. Bolton said in an e-mail.
The issue of the loophole for the S-300 sale already has caught the attention of some members of Congress.
Rep. Mark Steven Kirk, an Illinois Republican running for the Senate, is circulating a letter calling on Mr. Obama to close the loophole.
"We are deeply concerned that air defense and anti-ballistic missile systems, such as the Russian S-300, are excluded from the U.N. Register on Conventional Arms," the letter says.
"As you know, Russian transfer of the S-300 to Iran would undermine our national security, change the security dynamic of the region and undermine our diplomatic efforts," the letter states. "We urge you to demand the inclusion of this system in the list of prohibited transfers to Iran under the UNSC resolution."
The draft resolution is not likely to change; however, diplomats at the United Nations are working on the details of annexes to the draft that list specific Iranian entities and people who will be sanctioned.
U.S. officials said the Islamic Revolutionary Guard Corps, Iran's Islamist shock troops, will be a key target of sanctions.
Russia and Iran completed the S-300 deal in December 2007, and it is worth an estimated $800 million, according to an April 1 Congressional Research Service Report. The missile systems were to be delivered in March 2009 and operational by June 2009.
The missile system has a sophisticated radar and sensor system that allows it to track scores of missiles and aircraft and fire more than two dozen missiles. The missiles also can hit targets as high as 88,000 feet.
Russia in 2007 supplied Iran 30 Tor M1 anti-aircraft-missile systems worth more than $1 billion.
"It is not surprising that Russia would only agree to the draft U.N. resolution as long as the resolution would not specifically ban the sale of the S-300 system," said Kenneth Katzman, the Congressional Research Service analyst who wrote the April 1 report.
"Russia has agreed to the sale and has put delivery on hold as Iran has come under international scrutiny for its nuclear program, but Russia certainly wants the option of being able to complete the sale at some point, in line with international law," he said. "I would not, however, interpret this to mean that delivery to Iran is imminent."
Ivan Oelrich, vice president of the Strategic Security Program at the Federation of American Scientists, said, "It appears air-defense missiles can be sold to Iran under the U.N. sanctions agreement."
State Department spokesman Ian Kelly told reporters in October that U.S. policy was "that we do not believe that this is the time to sell Iran this kind of sophisticated defense capability."
"The S-300 is a very advanced surface-to-air missile," he said. "And we've understood from the Russian government that they have no plans to ship this sophisticated system to Iran at this time."