Kabul, Afghanistan (CNN) -- NATO troops in Afghanistan launched their biggest offensive of the war early Saturday, attacking what they call the last Taliban stronghold in a war-scarred southern province.
Military officials said the offensive -- dubbed Operation Moshtarak -- got under way at 2 a.m. (4:30 p.m. ET Friday). Moshtarak, a Dari word for "together," symbolizes the fact that combined forces are serving alongside one another.
"Insurgents who do not accept the government's offer to reintegrate and join the political process will be met with overwhelming force," the International Security Assistance Force Joint Command said in a statement. "However, the strongest of measures will be taken to protect the civilian population."
Some of about 15,000 troops from the United States, United Kingdom, Afghanistan and Canada attacked Taliban targets in and around Marjah, a city of 80,000 to 100,000 residents, where the Taliban has set up a shadow government, coalition military authorities said.
By about 8 a.m. (10 p.m. ET Friday) two firefights had erupted between the Taliban and U.S. Marines in Marjah.
"Marjah is the last enemy sanctuary in the Marine area of operations," said Brig. Gen. Larry Nicholson, 2nd Marine Expeditionary Brigade-Afghanistan commanding general. "This operation is designed to reconnect the people of Marjah with the legitimate government of Afghanistan. We are fully partnered with the Afghan government for this operation, and we have the resources we need to be successful."
The Afghan government described the offensive -- carried out in central Helmand with the Afghan National Army, Afghan National Police, ISAF Regional Command (South), and the Helmand Provincial Reconstruction Team -- as "clearing" operations.
The coalition said its troops expected to confront up to 1,000 entrenched Taliban fighters. It said they also expected foreign Taliban fighters to battle to the death but were prepared for local Taliban members in Marjah to try to escape.
"We will follow the enemies and bring them to justice," said Gen. Mohiyiden Ghori of the Afghan National Army.
In the past few days, forces from Afghanistan, Britain and other nations conducted air and ground operations to prepare for the assault and dropped leaflets in and around Marjah warning residents not to allow the Taliban to enter their homes.
The allies had been unusually vocal in describing their plans for the assault.
"I think there's a certain strength in the Pashtunwali culture just from laying it out there in saying, 'Hey, we are coming. Deal with it,'" U.S. Marine Gen. Larry Nicholson said.
Some of the 30,000 additional U.S. troops that President Obama sent to Afghanistan were taking part in the fight.
The goal is to force the Taliban from Marjah to free the opium-rich province of Taliban influence and drug traffickers. It's an example of a U.S. strategy to focus on population centers and separate the Taliban from Afghan civilians.
"It's about the security of the population, not fighting down insurgent numbers," British Gen. Gordon Messenger has said.
About 3,000 U.S. Marines were involved in the fight.
The advance notice given to residents was expected to help avert civilian casualties, a problem that has hurt the military's credibility among Afghans. They were also trying to get those Taliban who aren't hard-core to turn in themselves and their weapons.
Reaching the battleground was expected to be a big challenge for NATO and Afghan troops. The tough terrain is hard for tanks to traverse.
The town of Marjah is surrounded by roadside bombs, military officials said.
They said the Taliban has had months to plant the bombs, most of them homemade mixes of ammonium nitrate, shrapnel fuel, salt or flour.
Such bombs -- which are detonated remotely or by pressure plates -- have caused about 80 percent of the deaths in fighting in Helmand province, military officials said.
"This is possibly the largest IED threat NATO has ever faced," Nicholson has said.
Massive armored vehicles, called assault breacher vehicles, were to lead the charge into Marjah, coalition authorities said before the offensive.
The tank-like vehicles can destroy roadside bombs. Even with their help, though, military officials have increased staff at the hospital at Camp Bastion, in the capital of Helmand province, in anticipation that roadside bombs would cause casualties.
Troops were expecting to encounter booby-trapped houses and fierce urban combat.