(CNN) -- Four consecutive days of shelling and bombing by Syrian government forces into the besieged city of Homs have left residents cowering, afraid to escape and fearing for their lives.
That's how activists in Homs described the situation Wednesday. One, identified for his safety only as Abu Rami, said he heard explosions every few minutes from bombs launched by unseen forces outside the city limits.
Not even infants or medical crews have been spared, he said, calling it "a huge crime against humanity."
"They are shelling from a far distance," Abu Rami said. "They are using many kinds of weapons -- heavy weapons, anti-aircraft, they are using nail bombs."
At the United Nations, Secretary-General Ban Ki-moon condemned the "appalling brutality" of the bombardment of Homs, a city of about 1 million people. But he warned it would be "a grim harbinger of worse to come" without united pressure on Syrian President Bashar al-Assad's government to halt the bloodshed.
Ban said Arab League Secretary-General Nabil Elaraby plans to send a renewed Arab League monitoring mission back into Syria after it was suspended last month, when violence escalated. He said Elaraby had suggested beefing up the mission with U.N. observers -- a suggestion Ban said he raised in the Security Council and will discuss further "in the coming days."
Abu Rami said more than 60 people died Wednesday in Homs, including women and children and five infants who died at a hospital because the electricity was cut. Medical conditions are worsening too, he said.
"Yesterday they targeted the field hospital in Baba Amr (a Homs neighborhood) and they killed three doctors of this hospital," Abu Rami said. "We have a shortage of medical tools and medical supplies.
"We have at this moment more than 100 wounded people. We can't rescue them or make for them any necessary assistance."
Families who tried to escape from Baba Amr were captured by government forces and killed, he said.
The medical charity Medecins Sans Frontieres, also known as Doctors Without Borders, said Wednesday the Syrian regime was attacking the wounded and the staff who were treating them.
"In Syria today, wounded patients and doctors are pursued, and risk torture and arrest at the hands of the security services," said Marie-Pierre Allie, president of the group. "Medicine is being used as a weapon of persecution."
Patients are using false names and doctors are giving false diagnoses in attempts to elude security forces, who search for patients whose wounds are consistent with those inflicted during demonstrations, she said.
CNN cannot independently confirm reports from either side in Syria because the government has restricted journalists' access to the country.
Another activist in Homs, identified for his safety only as Danny, told CNN via a satellite Internet connection that people are scared to leave their homes because of snipers. Those who venture out tend to cling to walls or move through alleys to avoid the gunfire, he said.
It doesn't always work. "I have lost more than 30 of my friends," Danny said. "Ten or 12 of them died right in front of me 'cause I couldn't take them to the hospital, because I couldn't move them from the street."
He held up a rocket that he said landed on a house and a mortar bomb that he said had hit another house and killed a 2-year-old.
Danny said he is living in a house with about 20 other people, armed with only two handguns. Some of the bodies he had seen, he said, bore signs of electrocution; others had been cut in pieces.
The Syrian government has said it is fighting armed gangs and terrorists in its crackdown, which has continued unabated for 11 months. But the rebel Free Syrian Army is unable to fight back because government forces are striking from a outside the city, Abu Rami told CNN.
"Who's going to resist? You can't see anything, so there is not any armed resistance in these areas," he said.
Even at home, activists say, no one is safe. Government troops are moving from house to house by breaking through walls rather than venturing back onto the street, where they might face Free Syrian Army forces, Danny told CNN.
Syria has been ruled by the al-Assad family since 1970. When last year's "Arab Spring" protests spread to the country in March, the government responded with escalating force, but has struggled to put down what is now a widespread uprising.
The Syrian National Council, the major opposition umbrella group, repeated its call for outside intervention to halt the killing and lift the siege of cities like Homs. And Amnesty International called on Russia "and other countries with influence over Syria" to take action to end the assault on Homs, where they said the majority of the dead were unarmed civilians.
In particular, Amnesty urged Moscow "to make it clear to the Syrian government, both publicly and in private, that the military assault on the city of Homs must end immediately."
"The situation in Homs is critical, and is turning into a major humanitarian crisis. Russia has blocked international efforts to stop the massive human rights violations in Syria, stating that they have a better plan for resolving the crisis," Salil Shetty, Amnesty International's secretary-general, said Wednesday in a statement.
Russia, which joined China in vetoing a proposed U.N. resolution aimed at halting the crackdown, has stood by its Soviet-era ally. Russian Foreign Minister Sergey Lavrov touted Syrian President Bashar al-Assad's commitment to ending the violence during a visit to Damascus on Tuesday, and Prime Minister Vladmir Putin said Monday the situation is for the Syrians to resolve.
"One cannot act like an elephant in a china shop. People should be allowed to decide their future themselves," Putin said. "The conflicting sides and people should decide their future themselves."
Navi Pilay, the U.N. high commissioner for human rights, urged the international community Wednesday "to cut through the politics and take effective action to protect the Syrian population." She said thousands of demonstrators and civilians have been killed, injured, detained or tortured since the uprising began -- actions that may constitute crimes against humanity, punishable under international law.
U.N. officials estimate 6,000 people have died since protests began nearly a year ago. The Local Coordination Committees, a network of opposition activists that organizes and documents protests, puts the toll at more than 7,300.