Syrian security forces have launched a major assault on Hama, the country's third-largest city, a day before the start of the Islamic holy month of Ramadan.
The National Organisation for Human Rights, a Syrian activist group, said 136 people were killed in Hama and three other centres across the country on Sunday.
The activist group reported at least 100 deaths in Hama, after tanks and soldiers stormed the city and said that dozens of anti-government protesters died in Deir ez-Zor, Harak and Al Bukamal.
Al Jazeera's Rula Amin said that the number of deaths "are on the rise and the government seems adamant in wanting to crush the protests".
"We have seen that the government had intensified its effort to try and end the wave of protesters or contain it over the past two weeks using more severe measures, like mass arrests of activists as well as just regular people who took part in the protests, or sometimes never even went to the streets.
"So it seems that the government is expecting some sort of escalation in the protest movement during the month of Ramadan and they are trying to pre-empt it," our correspondent, reporting from neighbouring Lebanon, said.
The Syrian government has banned Al Jazeera and many other foreign media outlets from reporting from inside the country, making it difficult to verify reports of fighting.
US President Barack Obama said he was appalled by the Syrian government's use of violence against its people and promised to work with others to isolate President Bashar al-Assad.
"The reports out of Hama are horrifying and demonstrate the true character of the Syrian regime," Obama said in a statement released by the White House on Sunday.
JJ Harder, the press attaché of the US embassy in Damascus, told Al Jazeera that the Syrian government "has shown itself with its back against the wall".
"It is resorting to some desperate last ditch attempt, trying to save itself. It is full-on warfare on its own people," he said.
Asked if he accepted the Syrian government's contention that its forces were up against armed gangs, Harder said: "The Syrian government is completely delusional. They are making up fanciful stories that no one believes.
"Our ambassador Robert Ford was in Hama earlier this month, and he saw with his own eyes the violence that they are talking about. There was none.
"He maybe saw one teenager with a stick at a checkpoint, and the government is going on with these absolute fabrications about armed gangs running the streets of Hama and elsewhere.
"Hama has shown itself to be a model of peaceful protest. That was why our ambassador chose to go there.
"This is a coherent attack by the Syrian government. I think they must be thinking that the international community is not paying attention. But I think they are dead wrong," Harder said.
One of the Hama residents, a doctor who did not want to be further identified for fear of arrest, told Reuters that the tanks were attacking from four different direction and "firing randomly".
"They are firing their heavy machine guns randomly and overrunning makeshift road blocks erected by the inhabitants," he said by phone, the sound of machine gun fire crackling in the background.
He said that there were 51 people wounded at Badr hospital alone, which was running short of blood for transfusions. He said tanks had surrounded another main hospital, al-Horani.
Another resident said snipers had climbed onto the roofs of the state-owned electricity company and the main prison, and that electricity had been cut in eastern neighbourhoods.
In-depth coverage of escalating violence across Syria
Assad is attempting to crush an uprising against his 11-year rule that broke out in March, inspired by "Arab Spring" revolutions in Tunisia and Egypt, and has spread across the country.
Hama was the scene of a massacre in 1982 when Assad's father, the late president Hafez al-Assad, sent his troops to crush an Islamist-led uprising, razing whole neighbourhoods and killing up to 30,000 people in the bloodiest episode of Syria's modern history.
An activist group, Avaaz, said in a report last week that Syrian security forces had killed 1,634 people in the course of their crackdown, while at least 2,918 had disappeared.
Another 26,000 had been arrested, many of whom were beaten and tortured, and 12,617 remained in detention, it said.
Syria-based rights activist Mustafa Osso said the regime had detained more than 4,000 people in raids over the past week as it tries to contain the situation.
"The regime is trying to suppress the uprising because it knows that the opposition will try to hold protests between sunset and sunrise," Osso said.
He called the week of raids the "strongest security campaign since the uprising began".
'Tribe leader arrested'
Meanwhile, Syrian forces have arrested Sheikh Nawaf al-Bashir, head of the main Baqqara tribe in the restive Deir ez-Zor province, opposition sources told Reuters on Sunday.
Secret police agents arrested Bashir, who commands the loyalty of an estimated 1.2 million Baqqara, in Ein Qirsh district of Damascus on Saturday afternoon, they said. He is a prominent figure in the campaign against President al-Assad in the region.
Hours before his arrest Bashir told Reuters he was striving to stop armed resistance to a military assault on the provincial capital of Deir ez-Zor and to convince inhabitants to stick to peaceful methods, despite the killings by security forces of at least six civilians late on Saturday.
Deir ez-Zor, the capital of a province that carries the same name, is an oil hub but also one of the country's poorest regions.
Rami Abdul-Rahman, the director of the London-based Syrian Observatory for Human Rights, said about 60 vehicles filled with reinforcements from the northern city of Raqqah reached Deir ez-Zor after noon on Saturday.
"Intense shooting could be heard in the western parts of the city," he told The Associated Press news agency by telephone.
"Residents have placed barriers in the streets in order to obstruct advancing troops."
The tribal region, which borders Iraq, has been witnessing intense protests, calling for the downfall of al-Assad's regime.