A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obama and a senator that tested positive for poisonous ricin and set the nation's capital on edge a day after the Boston Marathon bombings.

Paul Kevin Curtis, 45, was arrested at his apartment near the Tennessee state line east of Memphis, said FBI Special Agent in Charge Daniel McMullen. It wasn't immediately known where he was being held.

U.S. President Barack Obama met with Saudi Foreign Minister Saud al-Faisal Wednesday, in the latest of a string of talks with key players in the Middle East and the Gulf as he mulls policy on Syria.

Obama dropped by a meeting between Prince Saud and U.S. National Security Adviser Tom Donilon at the White House, using a diplomatic practice designed to satisfy protocol between leaders of different political rank.

A letter addressed to U.S. President Barack Obama has preliminarily tested positive for the deadly poison ricin, the Federal Bureau of Investigation said Wednesday.

In a statement, the FBI said the investigation into the letter sent to Obama and another sent to Senator Roger Wicker was ongoing, adding there was "no indication of a connection" to the Boston Marathon bombings.

The United States slammed North Korea's belligerence Thursday and pressed China to rein in its ally, as U.S. officials downplayed a chilling spy agency report that Pyongyang has a nuclear-armed missile.

The alarming assessment of the North's nuclear capabilities, revealed by a U.S. lawmaker at a congressional hearing, came as tensions on the Korean peninsula mounted over an expected missile launch by Pyongyang.

U.S. President Barack Obama paid homage on Tuesday to Memphis soul and the role the music style played in advancing civil rights at a star-studded White House concert celebrating the genre.

"In the sixties and seventies, Memphis knew its share of division and discord and injustice," Obama recalled. It was in the Tennessee city in 1968 that civil rights hero Martin Luther King Jr. was assassinated.

U.S. President Barack Obama accused Republicans of stooping to political stunts Monday to block gun reform, in a fervent appeal delivered close to the site of the Newtown school massacre.

At a critical moment for hopes for sweeping action to stem gun violence, Obama traveled to Hartford, Connecticut, not far from Sandy Hook Elementary School, where 20 children and six adults were gunned down in December.

President Barack Obama will make key concessions to Republican foes next week when he unveils his U.S. budget that proposes cuts to cherished entitlement programs, the White House said Friday.

Obama's fiscal blueprint slashes the deficit by $1.8 trillion over 10 years, in what a senior administration official described as a "compromise offer" that cuts federal spending, finds savings in Social Security and raises tax revenue from the wealthy.

The United States is pressuring China's new President Xi Jinping to crack down on the regime in North Korea or face an increased U.S. military presence in the region, The New York Times reported late Friday.

Citing unnamed administration officials, the newspaper said the recent U.S. exchanges with China included a phone call from President Barack Obama to Xi.

Syrian President Bashar Assad has warned that the fall of his regime would destabilize the region "for many years", as U.S. President Barack Obama announced talks with Middle East allies over the crisis.

"The whole world knows that if Syria is partitioned, or if terrorist forces take control of the country, there will be direct contagion of the surrounding countries," Assad said in an interview with two Turkish media outlets, video of which was posted on his Facebook page Friday.

President Barack Obama will host leaders from key U.S. allies Jordan, Turkey, Qatar and the United Arab Emirates in the coming weeks, amid turmoil in Syria, the White House said Friday.

The U.S. administration said Jordan's King Abdullah II will meet with Obama on April 26 for talks "Jordan's political and economic reforms, the humanitarian crisis in Syria, and additional regional issues of mutual concern."