John Kerry was sworn in Friday as America's secretary of state, less than two hours after Hillary Clinton swept out of office on a day marred by yet another attack on a U.S. diplomatic post.

Supreme Court judge Elena Kagan administered the oath of office to Kerry during a private ceremony on Capitol Hill. He was joined by his wife Teresa, daughter Vanessa, brother Cameron and his Senate staff.

The United States is increasingly concerned that "chaos" in Syria could allow Hizbullah to obtain sophisticated weapons from the Damascus regime, Defense Secretary Leon Panetta told Agence France Presse in an interview.

Speaking two days after Israel carried out a bombing raid on a military site outside Damascus, Panetta said Washington was worried Hizbullah could exploit the 22-month conflict in Syria.

The White House Friday defended its nominee for defense secretary, Chuck Hagel, after his perceived weak performance at a Senate confirmation hearing.

It also denounced Republicans who pressed Hagel hard at Thursday's session. Hagel has been nominated to replace Leon Panetta in President Barack Obama's second term.

The White House condemned a suicide bombing outside the U.S. embassy in Ankara on Friday as a "terrorist attack," but said it did not yet know who was behind it.

White House spokesman Jay Carney said "we strongly condemn" the attack, which killed a Turkish security guard and wounded several other people.

A suicide bomber suspected to be a militant from an outlawed leftwing group blew himself up at the U.S. embassy in Ankara on Friday, killing a Turkish security guard and wounding three other people, officials said.

The bombing at the entrance to the highly-fortified embassy in an upmarket area of the capital was the latest in a series of attacks on American missions in the Muslim world, highlighting the vulnerability of the country's 70,000 diplomats.

U.S. Vice President Joe Biden on Friday warned Iran that opportunities for diplomacy over its disputed nuclear program were not unlimited, as he kicked off a three-country European tour.

In an interview with the German Sueddeutsche Zeitung daily, Biden said the "burden of proof" that Iran was not seeking a nuclear weapon lay with the authorities in Tehran.

A federal jury found a man guilty of federal terrorism charges on Thursday, rejecting the defense team's argument that Mohamed Mohamud was entrapped or induced by a yearlong FBI sting that began to target him when he was a teenager.

Mohamud was accused of leading a plot to detonate a bomb at Portland's 2010 Christmas tree-lighting ceremony. But the device he thought was a bomb was a fake, supplied by undercover FBI agents posing as members of al-Qaida.

Iran intends to install more modern equipment at Natanz, one of its main nuclear sites, according to a document seen by Agence France Presse on Thursday.

The U.N. atomic agency document said that Iran informed it in a letter dated January 23 that "centrifuge machines type IR2m will be used in Unit A-22" at the Fuel Enrichment Plant (FEP) at Natanz.

Five people were injured Wednesday in a shooting at an office building in Phoenix, Arizona, police and local media reported.

It was not immediately clear if the gunman was in custody, according to the Arizona Central news website. The local ABC television affiliate reported meanwhile that three people had been hit by gunshot and two others injured.

U.S. President Barack Obama's popularity has hit 60 percent, the highest level since he first took office four years ago, according to a poll released Wednesday.

The survey by ABC television and The Washington Post was made public just a little more than a week after the president's formal swearing-in was witnessed by an estimated one million people in Washington and millions more across the nation.