U.S. President Barack Obama told German Chancellor Angela Merkel on Thursday that the United States and its allies should prepare new sanctions against Russia if it escalates the crisis over Ukraine.

"The president underscored the need for the United States, European Union and other global partners to be prepared to meet further Russian escalation with additional sanctions," the White House said in a statement about the phone call.

President Barack Obama vowed to do more to protect U.S. soldiers on Wednesday as he paid a somber tribute to victims of last week's Fort Hood shooting, the second gun rampage at the Texas military base in five years.

Speaking before the families of three soldiers killed by a comrade who took his own life during the April 2 shooting, Obama referenced the 2009 massacre at the base which claimed 13 lives.

The official death toll from a huge U.S. landslide rose to 34 on Tuesday as the White House announced that President Barack Obama will visit the scene later this month.

Up to 12 people are still believed missing after a wall of mud and debris crashed down on the small town of Oso in the forests of Washington state on March 22.

President Barack Obama will travel on Wednesday to a Texas military base to honor the victims of a deadly shooting by a troubled U.S. soldier, the White House said.

Presidential adviser Dan Pfeiffer told CBS television on Sunday that president and First Lady Michelle Obama will attend a memorial service at the Fort Hood army base for three soldiers who died last week when Army specialist Ivan Lopez, 34, opened fire at the sprawling military facility.

Leading candidates in Afghanistan's presidential election voiced concern Sunday that voting was tainted by fraud, a day after millions defied Taliban threats and turned out to choose a successor to President Hamid Karzai.

World leaders praised the courage of Afghan voters, who cast their ballots in force despite bad weather and the violent campaign of intimidation, and urged patience in the long vote count.

United States lawmakers are mulling to toughen economic sanctions against Hizbullah, media reports said on Saturday.

A U.S. House Foreign Affairs Committee is set to introduce new legislation that would “target Hizbullah's fundraising activities,” the Israeli Ynet news reported.

President Barack Obama cleared the way Thursday for U.S. sanctions against anyone on either side of South Sudan's civil war who targets U.N. peacekeepers, threatens peace moves or abuses human rights.

The president issued an executive order allowing the use of property and asset seizures and immigration and visa bans against government or rebel officials who are judged at a later date to have transgressed.

The U.S. Supreme Court on Wednesday removed the overall limit on contributions a donor can make to political candidates, a move that could open the floodgates to campaign funding by rich individuals.

The move was immediately hailed by Republicans, who are gearing up to challenge President Barack Obama's Democratic supporters for control of the Senate in October polls.

Russian President Vladimir Putin called his American counterpart Barack Obama Friday to discuss a U.S. proposal for a diplomatic end to the Ukraine crisis while insisting to the United Nations that Moscow had "no intention" of further military action.

The White House said Putin phoned Obama following a bid presented to Russian Foreign Minister Sergei Lavrov by U.S. Secretary of State John Kerry earlier this week.

U.S. President Barack Obama sought Friday to allay Riyadh's criticism of his policies on Syria and Iran, telling the Saudi king their two countries remain in lockstep on their strategic interests.

He also assured King Abdullah that the U.S. "won't accept a bad deal" with Iran, as global powers negotiate a treaty reining in Tehran's controversial nuclear program.