BEIRUT — Syrian tanks and troops massed Monday outside the resistance stronghold of Homs for a possible ground assault that one activist warned could unleash a new round of fierce and bloody urban combat even as the Red Cross tried to broker a cease-fire to allow emergency aid in.

A flood of military reinforcements has been a prelude to previous offensives by President Bashar Assad’s regime, which has tried to use its overwhelming firepower to crush an opposition that has been bolstered by defecting soldiers and hardened by 11 months of street battles.

“The human loss is going to be huge if they retake Baba Amr,” said Rami Abdul-Rahman, who heads the Britain-based activist group Syrian Observatory for Human Rights.

The central city of Homs — and in particular the opposition district known as Baba Amr — has become a critical ground for both sides.

The opposition has lionized it as “Syria’s Misrata” after the Libyan city where rebels fought off a brutal government siege. Assad’s regime wants desperately to erase the embarrassing defiance in Syria’s third-largest city after weeks of shelling, including a barrage of mortars that killed up to 200 people earlier this month. At least nine people were killed in shelling Monday, activists said.

Another massive death toll would only bring further international isolation on Assad from Western and Arab leaders.

“The massacre in Syria goes on,” said U.S. Sen. John McCain during a visit to Cairo, where he urged Washington and its allies to find way to help arm and equip Syrian rebels.

McCain, a senior member of the Senate Armed Service Committee, said he did not support direct U.S. weapons supplies to Syrian opposition forces, but has suggested the Arab League or others could help bolster the fighting power of the anti-Assad groups. The U.S., he said, could assist with equipment such as medical supplies or global positioning devices.

“It is time we gave them the wherewithal to fight back and stop the slaughter,” he said.

Assad’s fall also would be a potentially devastating blow for his close ally Iran, which counts on Syria as its most reliable Arab ally and a pathway for aid to Tehran’s patron Hezbollah in Lebanon. But McCain urged for “like-minded” Western and Arab nations also to guard against attempts by al-Qaida or other extremists to exploit a leadership vacuum if the regime crumbles.

“For us to sit back and do nothing while people are being slaughtered … is an affront to everything America stands for and believes in,” said McCain, suggesting that the Republicans could seek to make Syria a central campaign issue in this year’s U.S. presidential election.

In Cairo, Arab League Secretary-General Nabil Elaraby suggested at a news conference in Cairo that Russia and China – two countries that recently supported Damascus by vetoing a U.N. Security Council resolution condemning Assad’s regime – may be shifting their positions.

“There are some indications, especially from China and to some degree from Russia that there may be a change in their stance,” he said, without elaborating.

Syria-based activist Mustafa Osso told The Associated Press that Assad’s military should face strong resistance as residents plan to fight until “the last person.” He added that Homs is facing “savage shelling that does not differentiate between military or civilians targets.”

The Baba Amr neighborhood on Homs’ southwest edge has become the centerpiece of the city’s opposition. Hundreds of army defectors are thought to be taking shelter, clashing with troops in hit-and-run attacks.

Amateur videos posted online showed what activists said were shells falling into Baba Amr. Black smoke billowed from residential areas. Phone lines and Internet connections have been cut with the city, making it difficult to get firsthand accounts from Homs residents.

In Geneva, a spokeswoman for the International Committee of the Red Cross said the group has been in talks with Syrian authorities and opposition groups to negotiate a cease-fire in the most war-torn areas.

“We are currently discussing several possibilities with all those concerned, and it includes a cessation of fighting in the most affected areas,” the spokeswoman, Carla Haddad, told the AP.

She said the talks weren’t aimed at resolving any of the entrenched political differences. “The idea is to be able to facilitate swift access to people in need,” Haddad said.

Clashes between military rebels and Syrian forces are growing more frequent and the defectors have managed to take control of small pieces of territory in the north as well as parts of Homs province, which is Syria’s largest stretching from the border with Lebanon in the west to Iraq and Jordan in the east. Increasingly, Syria appears to be careening toward an all-out civil war.

Activists believe Assad may be trying to subdue Homs — an important stronghold for anti-Assad groups — before a planned referendum Sunday on a new constitution. The charter would allow a bigger role for political opposition to challenge Assad’s Baath Party, which has controlled Syria since a 1963 coup.

But the leaders of the uprising have dismissed the referendum as an attempt at superficial reforms that do nothing to crack the regime’s hold on power.

“We have called for a boycott of the referendum which cannot be held while parts of Syria are a war zone,” said Omar Idilbi, a Beirut-based member of the opposition Syrian National Council.

The U.N. last gave a death toll for the conflict in January, saying 5,400 had been killed in 2011 alone. But hundreds more have been killed since, according to activist groups. The group Local Coordination Committees says more than 7,300 have been killed since March of last year. There is no way to independently verify the numbers, however, as Syria bans almost all foreign journalists and human rights organizations.

In the western Hama province, troops backed by armored personnel carriers and military buses stormed several villages, conducting raids and arrests. A 32-year-old man was killed by gunfire from a security checkpoint in the area, activists said.

In the Damascus suburb of Douma, thousands of people took part in the funeral of 19-year-old army conscript, Omar Halbouni, whom activists say was executed by the military because he refused to open fire at protesters in Homs a day earlier. It was the third straight day of funeral marches and protests in the tightly controlled regime stronghold.

“Freedom, freedom!” shouted mourners at his funeral, as they paraded his body strewn with flowers on a stretcher near the Abdel Raouf Mosque, said witness Mohammad al-Saeed.

By evening, a few hundred Syrians held an anti-Assad protest in Damascus only few hundred yards away from a major security building. Amateur video posted online showed men and woman shouting: “We will kneel only before God” and other slogans in support of Homs and other Syrian cities.

On Sunday, activists said at least 18 people were killed in Syria, including a senior state prosecutor and a judge who were shot dead by gunmen in the restive northwestern province of Idlib.