Two American soldiers were killed in a Taliban attack on a US base in relatively quiet eastern Afghanistan on Saturday, far from the front lines of a major US offensive in southern Helmand province, an extremist stronghold.

The surprise attack took place in the Zerok district of Paktika province, hundreds of miles from where US forces expected to engage the Taliban, highlighting the group's resilience in the face of US efforts to stabilize the country in the run up to the presidential election in August.

The Los Angeles Times reports that the base came under a "sophisticated" and "multi-pronged" attack from as many as 100 militants armed with rockets, mortars, white phosphorus, and a truck bomb. The Times says the attack "[forced] the defenders to call in airstrikes to avoid being overrun."

It is unclear how many US and Afghan troops were injured or how many insurgents were killed. The Associated Press reports that seven US and two Afghan soldiers were wounded and 30 militants killed, but the LA Times reports that US officials "declined to reveal the number."

In Helmand, the US push to extend the reach of the Afghan central government "has faced generally light resistance," reports Agence France-Presse. But one company of Marines has been in constant battle with insurgents since Thursday, when they seized a key canal and road junction at Mian Poshteh.

The 200 Marines fighting to hold the position arrived at dawn on Thursday, and they were still engaged in fierce combat through the weekend, Major Dan Gaskell told AFP at nearby Camp Delhi. "Echo company landed by the canal intersection and set up shop," he said late Saturday. "They have been fighting to hold that position.

"The enemy really wants it back, and have been doing everything they can to dislodge Echo. That continues."

The US has called in helicopter gunships three times to help the Marines, Gaskell said, including one attack using a Hellfire missile.

He said about 40 Taliban fighters were using small arms, rocket-propelled grenades and rockets against the Marines, who have based themselves in a walled compound.

Helmand is "criss-crossed" with canals like the one at Mian Poshteh, says AFP. The canals were built with US aid money to boost agriculture in the 1950s and 1960s. Today, the region is a center of cultivation, but for a crop the US probably did not have in mind: opium.

Helmand is a center for the opium trade, says the Pakistani daily newspaper, Dawn. Opium is a major breadwinner for the Taliban. Dawn says the province's financial significance to the group is just one of several reasons why Operation Khanjar is so important to the war effort:

Begun on Thursday, the operation is significant in many ways. One, it is the first major offensive after Barack Obama moved into the White House and appears to reflect his keenness to turn the tide of a war that has dragged on for nearly eight years without producing any significant results. Two, it aims at capturing the Taliban heartland which — and this is the third point — is a poppy-growing area. Helmand's capture could be a blow to the Taliban, for Afghanistan's booming drug trade is the biggest source of funding and arms for the militants. Crop destruction has failed to work so far.