MANAGUA - Forces loyal to Nicaraguan President Daniel Ortega launched an attack on the opposition stronghold of Masaya on Tuesday, as international calls mounted for an end to months of deadly violence in the central American country.

Around 40 vans full of heavily armed riot police and paramilitaries entered Masaya - southeast of the capital Managua - from four sides, according to images posted by residents on social media.

Automatic arms fire and warning sirens rang out in the flashpoint city's Monimbo neighborhood - the epicenter of recent protests calling for Ortega, who has dominated Nicaraguan politics for decades, to step down.

"They're attacking Monimbo! The bullets are reaching the Maria Magdalena parish church, where the priest is sheltered," archbishop Silvio Baez wrote on Twitter.

"May Daniel Ortega stop the massacre! People of Monimbo I beg you, save yourselves!"

More than 1,000 men firing machine guns entered the city of 100,000 people, residents said.

"They're attacking us from various entry points in Monimbo," Cristian Fajardo, a leader of the student protest movement, said in a WhatsApp message.

The US warned Ortega against pursuing the assault on Masaya and called for a halt to the deadly crackdown on anti-government protests that have left some 280 people dead over the past three months.

"We strongly urge President Ortega not to attack Masaya," tweeted Francisco Palmieri, the US principal deputy assistant secretary of state for Western hemisphere affairs.

"Continued gov't-instigated violence and bloodshed in #Nicaragua must end immediately. The world is watching."

Bursts of gunfire rang out over the sound of ringing church bells warning locals of the attack, witnesses told radio stations in Managua.

"They are shooting at houses in an irresponsible way. The message is that anyone who pops their head out will be killed: it's a message of terror," said Alvaro Leiva, secretary of the Nicaraguan Human Rights Association (ANPDH).

"What worries me the most is the children, pregnant women and old people."

The operation follows a bloody week in Nicaragua in which students and opposition groups came under attack several times amidst widespread civil action.

Ten days ago, at least 14 people died - four of them police or paramilitaries - after a raid by pro-government forces in the areas of Diriamba and Jinotepe, near Masaya.

Last week, five people - four of them police officers - died in clashes between opposition supporters and police backed by paramilitaries in the southeast town of Morrito while another operation by Ortega loyalists ended with at least 10 dead - including four police - on Sunday in Masaya.

On Monday, United Nations Secretary General Antonio Guterres urged an end to the violence and a resumption of the stalled dialogue with the opposition.

"It's absolutely essential that the violence ceases immediately and that the national dialogue is revitalized, because the only acceptable solution in Nicaragua is a political one," Guterres said in the Costa Rican capital San Jose.

In Brussels, Paul Oquist - the minister for national policies and one of the cabinet members closest to Ortega - told AFP in an interview that an attempted "coup" had been defeated.

"That is very good news because what it leaves then is dialogue, because the coup plotters did not want dialogue," Oquist said.

Opposition activists in Masaya and especially Monimbo have erected barricades to keep out government forces in and around their bastion.

The government launched a "clean-up operation" in July in a bid to break through the barriers and clear opposition activists from the streets.

Some 280 people have been killed in clashes in the impoverished central American country since protests erupted on April 18, initially against a now-ditched pension reform.

That ballooned into wider action against Ortega, his wife Vice President Rosario Murillo and the government, with the influential Catholic Church attempting to mediate.

The opposition accuses 72-year-old Ortega of running an oppressive regime and has demanded he resign and that elections scheduled for 2021 be brought forward to next year.

Ortega, a former guerrilla leader, has been in power since 2007 but also spent 11 years as president after the 1979 overthrow of US-backed dictator Anastasio Somoza.