After meeting little resistance in the first day of Operation Khanjar, or 'sword strike', he said units south of Garmsir were involved in heavy fighting.
American marines lost a single soldier during the first 24 hours of the 4,000-strong assault on the Taliban heartlands of southern Helmand province.
The operation is the first big push of President Barack Obama's surge strategy which has seen 21,000 reinforcements ordered to the country this year.
Brigadier General Larry Nicholson, commander of the 2nd Marine Expeditionary Brigade, said on Friday the 2/8 infantry battalion was meeting resistance at Toshtay, 16 miles south of Garmsir.
He said: "For 2/8, there is a hell of a fight going on in the southern quarter of the sector. 2/8 are going to face some challenges.
"An enemy-controlled baseline just south of Garmsir was crushed yesterday but that doesn't mean all the enemy have gone.
"In the next few days the enemy will observe us to see what we are doing. Then they will come back with a vengeance."
He added: "Nawa is quiet, too quiet. Something is eerie. The enemy has gone to ground, shuras [councils of elders] are being set up."
The American marine offensive entered its second day as British troops north of the provincial capital Lashkar Gah began a third wave of their own Operation Panchai Palang or Panther's Claw.
Around 800 Light Dragoons drove north after the Welsh Guards spent ten days capturing 13 crossings along the Shamalan canal a British military statement said.
On Friday it was announced Lt Col Rupert Thorneloe, the first British commanding officer to die in combat since the Falklands, had been killed by a roadside bomb during the operation.
The British said they hoped to allow free movement for next month's presidential elections by securing the road between Lashkar Gah and Gereshk.
American marines were ferried into Garmsir and Nawa districts by a fleet of dozens of helicopters and armoured convoys in the early hours of Thursday morning.
Khan Neshin, further south where Islamist militants were said to have set up a shadow government, was overrun later in the day.
A Taliban website, which in the past has exaggerated and fabricated coalition casualties, said 15 foreign and Afghan soldiers died on the first day of the assault.
A statement said: "In every locality they faced bloody attacks by the Mujahideen, as a result of which the enemy suffered heavy casualties.
It continued: "As a result, so far 15 foreign and internal enemy soldiers have lost their lives in the fighting." After an aggressive 36-hour opening phase, the operation was predicted to slow as American marines attempted to win local 'hearts and minds'.
First Lieutenant Kurt Stahl, a spokesman for the 2nd Marine Expeditionary Brigade, said: "When Marines go out into towns, they are always looking for opportunities to talk to village elders and explain why they are here."
"The intention is to understand each other, elders can express their concerns and an open flow of communication is secured."
