Five U.S. service members died Monday when a helicopter crashed in the Kandahar province of southern Afghanistan, officials said Tuesday.

The cause of the crash was under investigation, but a statement from the NATO-led International Security Assistance Force said "there was no enemy activity in the area at the time of the incident."

The names of the troops killed had not been released by U.S. officials Tuesday, but officials in the U.S. and Afghanistan said that all of those killed were American.

The helicopter went down west of the city of Kandahar, in the Daman district, coalition and Afghan officials said.

U.S. Air Force Lt. Col. Les Carroll, an ISAF spokesman, said the incident happened in darkness.

Earlier in the day, two U.S. service members and three Afghans were killed in a possible insider attack at a special forces site in Afghanistan. The gunman in that attack was dressed in an Afghan National Security Forces uniform.

The attack, which occurred shortly after newly appointed Secretary of Defense Chuck Hagel concluded his first trip to the country as head of the Pentagon, was just the latest in a string of bad news centered on the United States' relationship with Afghanistan.

Hagel's trip was marked criticism from Afghan President Hamid Karzai over the U.S. role in the country and the cancellation of a joint press conference due to "security concerns."

There have been 18 coalition deaths this year.

NBC News' Jamieson Lesko, Courtney Kube and Jim Miklaszewski contributed to this report.

Slideshow: Afghanistan: Nation at a crossroads

Rahmat Gul / AP

More than ten years after the beginning of the war, Afghanistan faces external pressure to reform as well as ongoing internal conflicts.

Launch slideshow

This story was originally published on Mon Mar 11, 2013 9:25 PM EDT

MUNICH - The United States is ready for direct talks with Iran if it is serious about negotiations, Vice President Joe Biden said late Saturday, backing bilateral contact many see as crucial to easing a dispute over Tehran's nuclear program.

Speaking at a security conference in Munich, Biden said Iran - which says it is enriching uranium for peaceful energy only - now faced "the most robust sanctions in history" meant to ensure it does not develop nuclear weapons.

"But we have also made clear that Iran's leaders need not sentence their people to economic deprivation and international isolation," Biden said. "There is still time, there is still space for diplomacy backed by pressure to succeed. The ball is in the government of Iran's court."

To date, fitful talks on Iran's nuclear program have been between Tehran and the EU's top diplomat representing six world powers including Washington. But analysts have suggested that with his re-election behind him, President Barack Obama might have more leeway to take on direct negotiations with Iran.

That makes the year ahead critical for chances of overcoming a stand-off which, if left to fester further, could see Iran approach nuclear weapons capability, possibly provoking military action by Israel that could inflame the Middle East.

Progress on Iran would also help ease regional tensions as the United States prepares to pull most combat troops out of Iran's neighbor Afghanistan by the end of 2014.

Asked whether Washington might consider direct talks with Iran to smooth the process, Biden said, "When the Iranian leadership, Supreme Leader (Ayatollah Ali Khamenei), is serious.

"We have made it clear at the outset that we would be prepared to meet bilaterally with the Iranian leadership, we would not make it a secret that we were doing that, we would let our partners know if that occasion presented itself.

"That offer stands, but it must be real and tangible and there has to be an agenda that they are prepared to speak to. We are not just prepared to do it for the exercise."

Negotiations with Iran have so far been overseen by European Union foreign policy chief Catherine Ashton on behalf of France, Britain, Germany, China, Russia and the United States. But they have made scant headway, raising fears Iran is simply playing for time while it develops nuclear know-how.

Many believe no deal is possible without a U.S.-Iranian thaw, requiring direct talks addressing myriad sources of mutual mistrust and hostility lingering since Iran's 1979 Islamic Revolution and the hostage crisis at the U.S. embassy in Tehran.

Iran has avoided direct, public talks with the United States, though some suggest Tehran would eventually welcome an opportunity to end its international isolation.

Speaking at a news conference in Munich, Republican Senator John McCain said he would have no objection to direct talks but questioned how much these would achieve if fundamental questions over Iran's nuclear program remained unresolved.

"I don't know when we will have direct talks between the United States and Iran. That is a subject for the president of the United States. I don't think anyone here objects to that," he said. But he added, "to have grounds for optimism, I think, would be a mistake."

Israel, which describes the prospect of Iran being able to weaponize enriched uranium as an existential threat, has made clear it would be ready to bomb the nuclear sites of its arch-enemy to prevent that outcome. The United States has also said it would not rule out the use of military force. 

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