Mexico City ? Farmers armed with machetes and homemade firebombs took three more hostages Saturday and demanded formal negotiations to end their violent standoff with the government over plans to build an airport on their land 18 miles northeast of Mexico City.

The showdown was in its third day with no sign of a breakthrough. Hundreds of protesters are now holding 15 captives in the tense, barricaded town of San Salvador Atenco, which is surrounded by about 900 riot police officers. Police have two protest leaders and 10 followers in jail.

Farmers block a road with machetes and Molotov cocktails, foreground, in San Salvador Atenco, outside Mexico City. Farmers protesting against the seizure of their land for a new Mexico City airport seized three more hostages on Saturday after threatening Friday to kill about a dozen hostages they had previously taken.

Federal authorities last October ordered 13,300 acres in the rural municipality expropriated for the $2.3 billion airport, setting off months of sporadic protests that exploded in violence late Thursday. The farmers seized government offices in the town, abducted other local officials, hijacked and burned vehicles, and attacked police with machetes. About 30 people were injured.

Protest organizers and officials of the state of Mexico have been on the phone at least five times since then, trying to work out an exchange of captives.

Those talks hit a snag Friday, despite jailed protest leader Ignacio del Valle’s telephone appeal to followers to accept the state’s offer. Under that proposal, del Valle and Adan Espinoza, who face charges of inciting violence, would stay in jail but the 10 other protesters and all the hostages would go free.

A rowdy assembly of farmers in the town’s auditorium shouted down the proposal. Participants said del Valle must have endorsed it under duress. “They’re torturing him,” one farmer shouted.

At a news conference late Friday, other protesters insisted again on the release of all 12 prisoners. They demanded that the federal government, not the state, take charge of the conflict and agree to formal negotiations through a mediator. They proposed three prominent human rights champions to play that role.

President Vicente Fox’s administration has tried to stay out of the conflict, saying the airport construction would continue as planned. However, Interior Minister Santiago Creel said federal officials were ready to negotiate with any group disposed to a peaceful settlement but would “act with a firm hand to avoid an increase of violence.”

About 34,000 people live in villages and farmland destined for the new six-runway facility that would replace Mexico City’s Benito Juarez International Airport.

Many object that the federal government’s offer to buy their land for 70 cents per square meter is too low, while others refuse to leave at any price.

The protesters took three more hostages Saturday, identifying them as undercover state police officers using fake credentials to pose as journalists.

For the second day in a row, leaders of the uprising brought hostages before TV cameras to show they had not been harmed. They include several policemen and state officials.