* Ryan says Obama foreign policy "unraveling"
* Biden hits Republican's "loose talk" on Syria, Iran
* Democrat pushes back on charges of U.S. weakness
By Andrew Quinn
WASHINGTON, Oct 12 (Reuters) - Vice President Joe Biden challenged his rival Paul Ryan on Thursday to show how a Republican administration would alter U.S. foreign policy in a dangerous world, but extracted few new details on issues including Iran, Syria and Afghanistan.
Biden and Ryan spent almost half the time in their only debate sparring over foreign policy, with each side hoping to score points on issues that have surged to the foreground as the U.S. presidential campaign tightens in its final weeks.
Republican challenger Mitt Romney has sought to portray President Barack Obama as a weak steward of U.S. power. Ryan accused the Democratic administration of leadership failures that led to incidents such as the attack last month on the U.S. mission in Benghazi, Libya which killed the American ambassador.
"What we are watching on our TV screens is the unraveling of the Obama foreign policy, which is making the world more chaotic and us less safe," Ryan said.
Republicans have seized on the Benghazi attack as emblematic of diminished U.S. standing in the world and accused the Obama administration of failing to provide sufficient information about the incident in a timely fashion.
"It took the president two weeks to acknowledge that this was a terrorist attack," Ryan said. "Look, if we're hit by terrorists we're going to call it for what it is, a terrorist attack."
While Ryan tried to depict the Obama administration as projecting weakness, Biden hit back hard with repeated questions on details of Romney's foreign policy plan, which Ryan was hard put to answer.
Biden mounted a tough defense of Obama, saying his decision to approve the killing of al Qaeda leader Osama bin Laden last year was a signal that ""if you do harm to America, we will track you to the gates of hell if need be."
SHORT OF WAR
Ryan described Iran as four years closer to a nuclear weapons capability than it was when Obama was elected, and called the president's policy of tightening sanctions on Tehran inadequate.
Biden noted that Obama had rallied major powers including Russia and China to support sanctions, and that neither Ryan nor Romney have yet described what more they would do short of war to persuade Iran to halt its pursuit of nuclear weapons.
"These are the most crippling sanctions in the history of sanctions. Period," Biden said. "There is nothing more that they say we should do than we have already done."
Analysts had expected a strong debate performance from the 69-year old Biden, a key Obama foreign policy advisor who grappled with global issues for years on the U.S. Senate Foreign Relations Committee.
Ryan, 42, is a seven-term congressman who aides say has been boning up on foreign issues as the campaign progresses.
On Afghanistan, Ryan said that it was crucial that the United States not lose gains it has achieved as it prepares to end the U.S. combat role and hand security responsibility to Afghan forces by the end of 2014.
Biden, declaring that the U.S. job in Afghanistan was almost done, said it was clearly time to wind up America's participation in the unpopular war.
"We are leaving. We are leaving in 2014. And in the process we're going to be saving over the next 10 years another $800 billion," Biden said.
He also accused the Romney-Ryan campaign of "loose talk" about Syria, saying the Republicans' repeated charge that Obama has not done enough to help topple President Bashar al-Assad was painting the United States into a dangerous corner.
"What more would they do other than put American boots on the ground? The last thing America needs is to get in another ground war in the Middle East," Biden said.
Ryan denied there were any proposals to send troops to Syria, but could not explain what more a Romney administration would do beyond tightening cooperation with regional allies and Syrian rebels - both of which the Obama administration has already done.