JUDY WOODRUFF: Now: Afghanistan and the deaths of six Americans, amid a renewed Taliban offensive. It happened near Bagram air field, outside Kabul, when a suicide bomber drove a motorcycle into a joint U.S.-Afghan patrol.

The Bagram attack was even worse than this one last August, when three American security guards died in a suicide attack in Kabul. And it came just three days after Secretary of Defense Ash Carter was in Afghanistan.

ASHTON CARTER, Defense Secretary: We have made gains that will put Afghanistan on a better path. More work lies ahead, and the national security of both our nations remains very much at stake. But we will succeed. The Taliban's advances in some parts of the country, even if only temporary, underscore that this is a tough fight, and it's far from over.

JUDY WOODRUFF: Indeed, Taliban fighters are pressing the Afghan army hard across the country. In late September, the militants managed a three-day takeover of Kunduz, a provincial capital in the north. And now government forces are falling back in Helmand Province in the south.

Officials there say the Taliban seized the strategically important Sangin district last night. More than 90 Afghan soldiers were killed in Helmand in two days of fighting. But some at least still sounded defiant.

MAN (through interpreter): The operation is going on by Afghan security forces in this area. The enemies cannot defeat us. We have a strong resolve to defeat them and defend our country, Afghanistan.

JUDY WOODRUFF: And they will have American help for a while longer. Two months ago, President Obama reversed course and announced that some 5,500 U.S. troops will stay in Afghanistan beyond 2016.

Late today, Defense Secretary Carter said the Bagram attack is a "painful reminder" of the dangers U.S. troops face in Afghanistan. And in a related development, the U.N. Security Council voted unanimously today to extend sanctions against the Taliban for 18 more months.

For more on the situation in Afghanistan, we turn now to freelance journalist Sune Engel Rasmussen in Kabul. I spoke with him a short while ago.

Thanks very much for joining us.

How did this suicide bomber get close enough to these American troops to kill them? What happened?

SUNE ENGEL RASMUSSEN, Freelance Journalist: Well, the American troops were carrying out a patrol on foot close to Bagram with a unit of Afghan national police. And as they were walking, the suicide bomber drove up to them on a motorbike laden with explosives and then rammed into them and detonated himself.

And that killed six American soldiers, is what we're hearing, and injured another three and injured three Afghan police. So, it didn't seem that difficult for him to get close to the soldiers apparently because they were walking on foot.

JUDY WOODRUFF: Is it common for U.S. or other NATO troops to be so vulnerable that way so close to this big air base?

SUNE ENGEL RASMUSSEN: Well, I don't know the details of how vulnerable they were, but I know they are conducting patrols, not just U.S. soldiers, but also soldiers from other nations around Bagram.

And this is something they have doing for a long time and will probably continue to do as part of their, train, advise and assist mission, where they go out with Afghans and assist them on the ground.

JUDY WOODRUFF: Well, we know that there are something like 9,800 U.S. troops in Afghanistan right now. What is their mission right now? Is it to defend? Is it to go after the Taliban? How would you describe it?

SUNE ENGEL RASMUSSEN: Well, NATO and the U.S. call it a train, advise and assist mission.

So, broadly speaking, that means training the Afghan security forces to take over security themselves. Advising is often also on the ground, for example, with Afghan special forces, when they try to the districts in, for example, southern Helmand province or in Kunduz, which fell a couple of months ago.

And then there's the assist part, which is a little more difficult to define exactly what that is, but that is, for example, fighting underground alongside Afghan troops. Now, the U.S. military says they don't conduct their own operation~s, but they are assisting the Afghans.

They're also carrying out airstrikes. They're also fighting a counterterrorism mission, as they say here. Now, that sometimes veers into what the rest of us might define as counterinsurgency, where they actually go out and they do patrols in villages that they think are influenced — or where there is a terrorism presence.

JUDY WOODRUFF: I ask because I think Americans don't hear as much about what's going on in Afghanistan. But you mentioned Helmand province in the south. The Taliban is putting up quite a serious fight there, aren't they?

SUNE ENGEL RASMUSSEN: They are. And they have been doing for quite a while now.

The Taliban launched what they call a spring fighting season in the spring, but it's been going pretty much continually since then for almost a year now. Sangin has been contested for a couple years actually, but it hasn't been as bad as we have seen over the past 48 hours now.

Lashkar Gah, the provisional capital in Helmand, looks seriously threatened now, as it hasn't done at any point actually during the war. The Taliban have managed to take a lot of districts surrounding the capital. And that's also why we have seen a recent arrival of both the U.S. and U.K. special forces in Helmand to help out the Afghans secure the province.

Eventually, the Afghans will have to take control of security themselves, but both the regular Afghan forces, but also the special Afghan forces, which are actually quite competent fighting, are stretched in the province, as it looks now.

JUDY WOODRUFF: Well, very tough, especially as we get so close to Christmas.

Sune Engel Rasmussen, we thank you.

SUNE ENGEL RASMUSSEN: You're welcome.