Hillary Clinton and Marco Rubio are early frontrunners to become the next president of the United States, according to a well-regarded poll released Thursday.

Clinton -- a celebrity former secretary of state, first lady and New York senator -- who is bidding to become America's first female president, is head-and-shoulders above her 2016 Democratic party rivals according to a Quinnipiac University poll.

A two-star naval officer and well-respected Pentagon spokesman is moving over to the State Department to brief reporters on U.S. foreign policy and diplomatic affairs, a U.S. official said Wednesday.

Rear Admiral John Kirby, who has been the highly-visible face of the Pentagon since December 2013, is to take over as State Department spokesman from Jen Psaki, a U.S. administration official confirmed to Agence France Presse.

The family of an unarmed African American teenager killed by a white police officer in Ferguson, Missouri said Wednesday it will sue the city for his "wrongful death."  

The family said in a statement they will formally announce a "civil suit against the City of Ferguson in the wrongful death of Michael Brown."

Chinese nuclear experts believe North Korea may already have a nuclear arsenal of 20 warheads and the uranium enrichment capacity to double that figure by next year, the Wall Street Journal reported Thursday.

The estimate, which the Journal said was relayed to U.S. nuclear specialists in a closed-door meeting in February, is significantly higher than any previously known Chinese assessment.

The U.S. Navy announced Wednesday that it had successfully completed the first in-flight refueling of a drone, widely increasing the use and range of the unmanned aircraft.

In a flight along the eastern coast of the United States, the unmanned prototype X-47B approached a tanker aircraft, managed to snag its refueling boom and received two tons of fuel, the U.S. Navy said.

The U.S. Senate unanimously passed legislation Wednesday aimed at stopping human trafficking, after lawmakers overcame a dispute about the measure's abortion language that prompted weeks of gridlock.

The partisan standoff had delayed a confirmation vote on President Barack Obama's attorney general nominee, federal prosecutor Loretta Lynch. That Senate vote is now expected as early as Thursday.

The United States accused Russia Wednesday of building up its forces along the border with Ukraine, boosting air defense systems in the country and training Ukrainian rebels in the east.

"Combined Russian-separatist forces continue to violate the terms of the 'Minsk 2' agreement signed in mid-February," acting State Department spokeswoman Marie Harf said in a statement.

Al-Mustaqbal Movement leader and former Prime Minister Saad Hariri on Wednesday kicked off a visit to Washington, where he announced that his political camp is trying to spare Lebanon any repercussions from the “drastic disputes” with Hizbullah.

“We must exert efforts to end the rift and elect a president for Lebanon,” said Hariri after talks with U.S. Secretary of State John Kerry in the U.S. capital.

French far-right leader Marine Le Pen, named by U.S. Time magazine as one of the world's 100 most influential people, said Tuesday it was recognition that her party was on the rise.

Le Pen said it showed the National Front was being taken seriously internationally and that one day the once fringe protest party founded by her ageing father Jean-Marie could take power.

The Afghan Taliban said their annual "spring offensive" will begin on Friday, vowing nationwide attacks in what is expected to be the bloodiest fighting season in a decade as NATO forces pull back from the frontlines.

This year's offensive marks the first fighting season in which Afghan security forces will battle the insurgents without the full support of U.S.-led foreign combat forces.