A car trailed an official motorcade that was carrying the daughters of U.S. President Barack Obama on Tuesday, sparking a brief lockdown at the White House.

The Secret Service, which is in charge of the president's security, said the vehicle followed the motorcade past a security barrier about 200 meters (650 feet) from the White House.

U.S. President Barack Obama on Tuesday described the kidnapping of more than 220 schoolgirls by Islamists in Nigeria as "heartbreaking" and "outrageous" as Washington deployed military experts in the hunt for the children.

Obama urged global action against Boko Haram and confirmed Nigerian leaders had accepted an offer to deploy US personnel there, soon after residents said the extremist group had seized eight more girls, aged between 12 and 15, again in the embattled northeast.

President Barack Obama is expected to meet Syrian opposition leader Ahmad Jarba during his visit to the United States this week, a senior U.S. official told Agence France Presse Tuesday.

Speaking on condition of anonymity, the official said "we expect that the president will meet" Jarba during his visit.

The United States secured long term access Monday to a military base in Djibouti that it uses to launch counter-terrorism missions, including drone strikes, in Yemen and the Horn of Africa.

U.S. President Barack Obama and his Djibouti counterpart Ismail Omar Guelleh announced the renewed "long term lease" on Camp Lemonnier to reporters as they met at the White House.

Thousands of American and Philippine troops launched large annual exercises on Monday after U.S. President Barack Obama vowed "ironclad" backing for its Asian ally, locked in a tense maritime row with China.

Filipino Foreign Secretary Albert del Rosario said the 10-day drills were necessary to deal with the challenge of "aggressive" neighbors intent on "changing the status quo".

Nigerian President Goodluck Jonathan on Sunday vowed to ensure the release of 223 schoolgirls abducted by suspected Islamists, saying he has sought help from U.S. President Barack Obama to overcome the nation's security challenges.

"We promise that anywhere the girls are, we will surely get them out," Jonathan said in a live broadcast on radio and television.

President Barack Obama offered his Afghan counterpart Hamid Karzai additional U.S. assistance Sunday to relief efforts after a landslide killed hundreds of people in northern Afghanistan.

Much of Aab Bareek village in Badakhshan province was swallowed on Friday by a fast-moving tide of mud and rock that swept down the hillside and left almost no trace of hundreds of homes. 

U.S. President Barack Obama and German Chancellor Angela Merkel admitted Friday that differences remained in a damaging row over U.S. spying which has strained their personal and diplomatic relationship.

Merkel on Friday made her first visit to the White House since revelations broke last year about National Security Agency (NSA) spies eavesdropping on her mobile phone, as well as about wider U.S. surveillance programs.

U.S. President Barack Obama said Friday the botched execution in Oklahoma of a convicted murderer who was left writhing in agony after having a lethal injection was "deeply troubling."

Obama argued there are some crimes so heinous that capital punishment could be merited, but warned the incident on Tuesday raised "significant questions about how the death penalty is being applied."

The United States and Germany delivered a firm warning to Russia on Friday that it would face direct and painful economic sanctions if Ukrainian elections later this month are disrupted.

U.S. President Barack Obama and German Chancellor Angela Merkel delivered the statement after talks at the White House appeared to bring the prospect of "sectoral" economic sanctions closer than ever before.