BEIRUT (AP) -- Between blasts of rockets and mortar fire, Syrians used loudspeakers to call for blood donations and medical supplies Thursday in the stricken city of Homs, where a weeklong government offensive has created a deepening humanitarian crisis.

Government forces are trying to crush pockets of violent resistance in Homs, the epicenter of an 11-month-old uprising that has brought the country ever closer to civil war. The intense shelling in restive neighborhoods such as Baba Amr has made it difficult to get medicine and care to the wounded, and some areas have been without electricity for days, activists say.

"Snipers are on all the roofs in Baba Amr, shooting at people," Abu Muhammad Ibrahim, an activist in Homs, told The Associated Press by phone.

"Anything that moves, even a bird, is targeted. Life is completely cut off. It's a city of ghosts," he added.

As he spoke, explosions could be heard in the background.

"The bombardment has not eased, day or night," he said, asking to be identified by his nickname for fear of reprisals. "Do you hear the sound of the rockets? Children have been wounded, elderly with extreme injuries."

Hundreds of people are believed to have been killed since early Saturday in the heaviest attack the city has endured since the uprising began in March, activists said.

"This brutal assault on residential neighborhoods shows the Syrian authorities' contempt for the lives of their citizens in Homs," said Anna Neistat, associate emergencies director at Human Rights Watch. "Those responsible for such horrific attacks will have to answer for them."

Human Rights Watch also said eyewitness accounts, as well as video reviewed by the group's arms experts, suggest Syrian government forces are using long-range, indirect fire weapons such as mortars.

Such weapons "are inherently indiscriminate when fired into densely populated areas," the New York-based group said.

The wounded have overwhelmed makeshift hospitals and clinics, and there were growing concerns that the locked-down city could soon run out of supplies.

"There is medicine in the pharmacies, but getting it to the field clinics is very difficult. They can't get the medicine to the wounded," Mohammed Saleh, a Syria-based activist, told the AP by telephone.

Baba Amr, he said, has been without electricity since Saturday.

The assault on Homs began after reports that army defectors and other armed opponents of President Bashar Assad were setting up their own checkpoints and taking control of some areas. The reports could not be confirmed.

But the city is the capital of Syria's largest province, stretching from the Lebanese border to the Iraqi frontier. If rebel forces keep gaining ground there, some believe they could ultimately carve out a zone akin to Benghazi in eastern Libya, where rebels launched their successful uprising against Moammar Gadhafi last year.

Saleh said most of the government attacks have been "bombardment from a distance," with regime forces keeping armored vehicles out of the neighborhoods.

Fighters from the rebel Free Syrian Army have been firing back with rocket-propelled grenades and rockets, according to activists' accounts.

The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees were trying to compile numbers and names of those killed Thursday. The LCC, an activist group, said up to 100 people were killed in Homs, but the toll was impossible to independently verify. The Observatory reported 63 deaths in Homs.

Activists also reported violence in the towns of Zabadani and Daraa.

As the bloodshed persists, the international community is searching for new diplomatic approaches to stop the protracted conflict.

The Syrian government blames the unrest on a foreign conspiracy by Israel and the West. It says armed gangs and terrorists are behind the uprising, not protesters seeking democratic change.

The uprising began with mostly peaceful protests but has transformed into an armed insurgency against Assad in many areas, raising fears the country is spiraling toward civil war. In January, the U.N. estimated an overall death toll of more than 5,400 since March.

The number of children killed has climbed into the hundreds, said Radhika Coomaraswamy, the U.N. representative for children in armed conflict, adding that the situation was particularly harrowing in Homs.

The Syrian regime's crackdown on dissent has left it almost completely isolated internationally as nations have imposed sanctions and withdrawn diplomats. In the latest action, Libya on Thursday gave Syria's top envoy to the country and embassy staff 72 hours to leave, according to Libyan Foreign Ministry press officer Saad Elshlmani.

Assad has political backing from Russia and China, which delivered a double veto over the weekend in the U.N. Security Council that blocked a resolution calling on him to leave power.

On Wednesday, U.N. Secretary-General Ban Ki-moon said the lack of unity on the council "has encouraged the Syrian government" to step up its attacks on civilians.

"Thousands have been killed in cold blood, shredding President Assad's claims to speak for the Syrian people," Ban said. "I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighborhoods, is a grim harbinger of worse to come."

The sanctions are crippling Syria's economy, but they have failed to stop the military offensives.

There also are fears that the conflict is taking on dangerous sectarian overtones in some areas, including Homs.

Syria's 22 million people are overwhelmingly Sunni Muslim, but Assad and the ruling elite belong to the Alawite sect, which comprises about 10 percent of the population.

The political domination by Alawites has bred seething resentment, which Assad tried to tamp down by enforcing the strictly secular ideology of his Baath Party.

But as the uprising surged, with Sunnis making up the backbone of the revolt, Assad called heavily upon his Alawite power base to crush the resistance, feeding sectarian tensions like those that fueled civil wars in Iraq and Lebanon.

A senior Arab League official said the Cairo-based organization will discuss Sunday whether to recognize the opposition Syrian National Council as the legitimate representative of Syria and whether to allow it to open offices in Arab capitals. The official spoke on condition of anonymity because no decision has been made on the issue.

The U.N. chief said the head of the Arab League plans to send observers back into Syria and has raised the possibility of a joint mission with the United Nations. Ban provided no specifics, but the idea appears aimed at giving the league a boost after its earlier mission was pulled out of Syria because of security concerns.

Also Thursday, Germany expelled four Syrian diplomats following the arrest this week of two men accused of spying on Syrian opposition groups in the country.

Foreign Minister Guido Westerwelle said he ordered the expulsions of the four Syrian Embassy employees.

German federal prosecutors said Tuesday they had arrested a Syrian and a German-Lebanese dual national on suspicion that they spied on Syrian opposition supporters in Germany for several years.

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Associated Press writers Lee Keath in Beirut, Geir Moulson in Berlin and Anita Snow at the United Nations contributed to this report.

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