WASHINGTON (AP) - President Barack Obama on Sunday heralded the release of Americans held prisoner in Iran and the full implementation of a historic nuclear accord with the Islamic Republic, holding both up as victories for "smart" diplomacy and his pledge to deal directly with enemies of the United States.

"This is a good day," Obama said in a statement from the White House. "When Americans are freed and returned to their families, that's something we can all celebrate."

The president spoke at the close of an extraordinary weekend of diplomacy that saw the back-to-back release of the five Americans and the lifting of billions in international sanctions on Iran as part of the nuclear accord.

Yet underscoring the strain that continues to exist between the U.S. and Iran, the Obama administration also announced new penalties Sunday on 11 individuals and entities involved in Tehran's ballistic missile program.

"We will continue to enforce these sanctions vigorously," Obama said. "We are going to remain vigilant about it."

For Obama, the diplomatic breakthroughs are a validation of his early promises to deal directly with nations such as Iran. The U.S. and Iran broke off diplomatic relations after the 1979 hostage-taking of the U.S. Embassy in Tehran.

"America can do - and has done - big things when we work together," Obama said. "We can lead this world and make it safer and more secure."

The nuclear talks have brought a sense of normalcy to relations with the U.S. and Iran, with top officials from each country in somewhat regular communication. But Obama emphasized that the U.S. continues to have deep concerns about Iran's destabilizing actions in the Middle East and its threats to Israel.

Obama spoke shortly after the Americans began their journey home. A charter plane left Tehran for Switzerland with the Americans - all four who had been detained, according to Iran state television, or only three, the U.S. said - as part of a prisoner swap.

Within hours of their release, the U.S. imposed sanctions against those involved in Iran's ballistic missile program as a result of Tehran's firing of a medium-range ballistic missile.
U.N. experts said in a report in December that the missile test in October violated sanctions banning Iran from launches capable of delivering nuclear weapons.
A U.S. Treasury official said Iran's ballistic missile program poses "a significant threat to regional and global security."

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