U.S. President Barack Obama arrived in Afghanistan late Tuesday on a surprise visit a year after American elite Navy SEALs killed al-Qaida chief Osama bin Laden in a raid deep inside Pakistan.

Obama was expected to sign a strategic partnership on future U.S.-Afghan ties once American troops have pulled out of Afghanistan in 2014 with Afghan President Hamid Karzai, White House officials told an Agence France Presse photographer.

U.S. President Barack Obama on Tuesday gave the U.S. Treasury authority to stop Syria and Iran wriggling out of sanctions by going after foreign firms, banks or individuals that ease their isolation.

The move, contained in an executive order, will allow the Treasury to publicly identify those engaging in "evasive and deceptive activities" and bar them from access to the U.S. financial and economic sectors, officials said.

Secretary of State Hillary Clinton headed to China Tuesday as U.S. officials raced to find a solution to a sensitive row over a top dissident reportedly holed up at the U.S. embassy in Beijing.

Clinton and Treasury Secretary Timothy Geithner had long planned to go to Beijing for the annual meeting between the world's two largest economies that now is likely to be overshadowed by the case of blind activist Chen Guangcheng.

Three U.S. Marines and a State Department employee were punished over their involvement with a Brazilian prostitute in December, U.S. Defense Secretary Leon Panetta said Tuesday.

The admission came as officials were still reeling from the fallout of a prostitution scandal implicating Secret Service agents and military personnel working on a visit to Colombia by President Barack Obama.

U.S. President Barack Obama on Tuesday commemorated the 1915 massacre of Armenians by Ottoman Turks, calling for "a full, frank, and just acknowledgment of the facts" of the "brutal" killings.

While denouncing the massacre of 97 years ago as "one of the worst atrocities of the 20th century," Obama did not use the term "genocide," but he implicitly called for Turkey to acknowledge its role.

Iran on Tuesday warned that new U.S. sanctions targeting its access to surveillance technology were "negative" and could adversely affect its crucial talks next month with world powers over Tehran's nuclear program.

"We consider any kind of sanction as negative and a wrong step," foreign ministry spokesman Ramin Mehmanparast said in his weekly briefing.

The White House has probed President Barack Obama's advance staff for any link to a Secret Service sex scandal in Colombia but found no evidence of "improper" conduct, a spokesman said Monday.

The clearing of White House staff who had gone to Cartagena to prepare for President Barack Obama's arrival came as the Pentagon said it had suspended the security clearances of 11 military personnel over the incident in the resort.

U.S. President Barack Obama said on Monday that the "killing of innocents" must end in Sudan and South Sudan, after weeks of border fighting in contested regions and tension between the two states.

"In Darfur, Abyei, South Kordofan and the Blue Nile, the killing of innocents must come to an end. The presidents of Sudan and South Sudan must have the courage to negotiate because the people of Sudan and South Sudan deserve peace," Obama said at the U.S. Holocaust Museum in Washington.

He has been mocked as rich and out of touch, struggled to woo his own party and left a trail of gaffes -- but Mitt Romney still has a genuine shot to make Barack Obama a one-term president.

A cascade of new polls, uncertain economic data and fresh concern that Europe's woes could cross the Atlantic, point to the Republican's best chance of beating Obama in November -- dismay about America's slow economic recovery.

U.S. President Barack Obama late Friday called for an end to fighting between Sudan and South Sudan and negotiations between the leaders of the two countries to settle their conflict.

"We know what needs to happen -- the government of Sudan must stop its military actions, including aerial bombardments," Obama said in a videotaped message to the people of the two countries.