President Barack Obama pressed opponents Saturday of a deal on Iran's contested nuclear program to be patient and not "screw up" the potential for a historic agreement.

In addition to staunch Republican opposition in the United States to an accord with the Islamic republic, Iran's Supreme Leader Ayatollah Ali Khamenei warned on Thursday that there were no guarantees of an agreement.

U.S. President Barack Obama announced Saturday that Brazilian counterpart Dilma Rousseff will visit Washington on June 30, almost two years after she canceled a trip over a U.S. spying scandal.

Obama made the announcement during a bilateral meeting with Rousseff on the sidelines of the Summit of the Americas in Panama City.

Hillary Clinton is expected to finally announce her candidacy for U.S. president Sunday, ending prolonged speculation that she once again seeks to become the first woman elected to the White House.

Seven years after her bitter nomination defeat to Barack Obama, the former secretary of state and one-time first lady would enter the race as the Democratic Party's overwhelming favorite, as Clinton and her rivals gird for a bruising, 18-month campaign slog.

U.S. President Barack Obama spoke with Venezuelan President Nicolas Maduro for the first time Saturday, telling him Washington was not seeking to threaten Caracas amid sky-high tensions.

U.S. officials said the two leaders met for a few minutes on the margins of the Summit of the Americas in Panama City, where Maduro used the stage to air his grievances about U.S. sanctions against Venezuelan officials.

U.S. President Barack Obama arrived home late Saturday, just hours after unprecedented face-to-face talks with Cuban President Raul Castro and a first meeting with Venezuelan President Nicolas Maduro.

Obama characterized his meeting with Castro -- the first sitdown between leaders of both nations since 1956 -- as "candid and fruitful," while the White House said the U.S. leader voiced support for a peaceful dialogue between Venezuela's government and the opposition during brief talks with Maduro.

U.S. President Barack Obama and Cuba's Raul Castro heralded a new era of bilateral relations on Saturday as they both addressed a landmark Summit of the Americas ahead of historic one-on-one talks.

Sitting around an oval table with some 30 other regional leaders in Panama City, Obama and Castro spoke one after the other in an unprecedented public exchange between the leaders of the Cold War-era foes.

Beijing hit back Friday at U.S. President Barack Obama's criticism of Chinese construction in the disputed South China Sea, arguing that it is Washington that has greater military "muscle".

The Chinese foreign ministry's retort came a day after Obama warned that Beijing was "using its sheer size and muscle to force countries into subordinate positions", amid reports of controversial Chinese land reclamation efforts.

Presidents Barack Obama and Raul Castro are expected to hold talks Saturday, a U.S. official said, raising the prospect of the first substantive meeting between an American and Cuban leader in more than five decades.

Obama and Castro are in Panama for the two-day Summit of the Americas, Cuba's first, raising expectations of a landmark follow-up to their historic announcement on December 17 that their countries would restore ties severed since 1961.

Foreign Minister Delcy Rodriguez said Wednesday she pressed a visiting U.S. diplomat over Caracas' wish to have Washington revoke an order calling Venezuela a national security threat.

Rodriguez, who met with Thomas Shannon, top counselor in the State Department, urged Shannon to have Obama revoke the order. 

Barack Obama became the first sitting U.S. president to set foot in Jamaica since 1982 on Wednesday -- and made some time for the late reggae legend Bob Marley.

On arrival, Obama descended from Air Force One to greet a long line of people including Jamaican Prime Minister Portia Simpson-Miller, whom he embraced.